Territorial evolution of the United States

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The United States of America was created on March 1, 1781, with the entry into force of the Articles of Confederation. In its history, it has gained independence from Great Britain, replaced its government with a new constitution, endured a civil war, and expanded across North America and the seas, growing from thirteen states to fifty states and many territories from the western Pacific Ocean to the Caribbean Sea.

Notes[edit]

  • "Unorganized territory" is not a name; it simply means Congress has not passed an organic act for the territory nor did it otherwise have a name. In most situations, the purpose of unorganized territory was to act as land for Native American settlement. However, the last mainland unorganized territory was indeed colloquially referred to as "Indian Territory." All land ceded to the federal government without an official name is labeled as "Unorganized territory".
  • This article includes all territories, unincorporated and incorporated, that the United States has possessed or claimed throughout its history, with the following notes:
  • Wartime occupations of U.S. territory are noted only when they involved a substantial replacement of the local government, or an entire territory surrendering. For example, during World War II the Philippines had a separate government administering it, while the captured Aleutian Islands were small military occupations. Likewise, the United States regaining lost territory is only noted if the territory was earlier noted as being lost, or if it is being returned following the end of hostilities, such as through a treaty; thus, the recapture of the Aleutian Islands during World War II is not noted, but the cession of islands off Maine after the War of 1812 is.
  • Islands claimed under the Guano Islands Act had varying amounts of control and claims from other countries; even though the U.S. and United Kingdom may have claimed the islands on paper, many were uninhabited and generally ignored once the guano miners left. The names used for guano islands are generally the modern names, as the islands could have many names based on multiple sightings and claims, and since for many they were uninhabited, there was no local name to fall back on.
  • The maps are concerned only with the official definition of the United States by itself, and any claims and controls by foreign powers, so proposed states or temporary territories, like State of Deseret, Jefferson Territory, and New Mexico under Stephen Kearny, are listed as changes, and the change is mapped, but omitted from the map of the country.
  • The following maps are supplied for each date, as relevant:
    • A map of the United States in central North America (what is currently the contiguous United States), as defined by the United States. This means the maximal borders are used in the case of a dispute. A map of the specific change is also included. Overlaps in definitions, active border disputes between parts of the U.S., or a disagreement between a part of the U.S. and the federal government over a part of the international border (the main example being when Arkansas Territory claimed area of Mexico without permission from the federal government), are noted here as red areas. If an area is ambiguous or challenged, but not actively pursued (like The Wedge), it is noted in the changes but not in the main map.
    • A map of the international disputes involving the United States in central North America. These maps will include the undisputed area of the country, as well as any area claimed by another, with certain rules: The other country must be internationally recognized, or be the sole civil government in any part of the U.S. to count, so micronations are excluded. A map of the specific change is also included.
    • A map of the United States in the Caribbean Sea; due to the lower complexity of these maps, they include international disputes.
    • A map of the United States in the Pacific Ocean, with the same rules as the Caribbean Sea.
    • A map of the United States in northwest North America, with the same rules as the Caribbean Sea.
  • Dates are given in local time. This only matters for some changes in the mid-Pacific; Guam, for example, surrendered on the morning of December 10, 1941, which would have been December 9 in the mainland United States.

Table of changes[edit]

Date Event Map Change
March 1, 1781 The Articles of Confederation entered into force, creating the United States of America.[1] The capital was not specifically established; at the time, the Congress of the Confederation met in Philadelphia.[2]

Many states had vaguely defined and surveyed borders; these are not noted as contested in the maps unless an there was an active dispute. The borders of North Carolina were particularly poorly surveyed. Its border with South Carolina was done in several pieces, none of which truly matched the spirit of the charter.[3][4] Its border with Virginia was surveyed by North Carolina only as far as the Cumberland Gap, but Virginia's survey went all the way to the Tennessee River. The two surveys were roughly two miles apart, creating a thin area claimed by both states. Since the dispute only technically went as far as the Cumberland Gap, both lines are presented past there, with North Carolina's faded to show that it was more an assumption. The border was supposed to follow 36°30′ north, but early surveying errors caused it to veer north of that, reaching a distance of 17 miles off by the time it reached the Tennessee River.[5]

Several northeastern states had overlapping claims; Connecticut, Massachusetts, and New York all claimed land west of their borders, overlapping with each other and with Virginia's sizeable claim. However, of these, only Connecticut's was seriously pursued. Virginia is considered to be the most legitimate claimant to the vast northwest, which they divided into counties and maintained some limited control.

Map of the United States in central North America from March 1, 1781, to October 29, 1782 Map of the change to the United States in central North America on March 1, 1781
The entirety of the new United States was claimed by Great Britain, and the northeastern area of New York was claimed by the Vermont Republic. Of particular note are Machias Seal Island and North Rock, two small islands off the northeast coast which remain disputed until the present day. Map of the international disputes involving the United States in central North America from March 1, 1781, to April 4, 1784
April 4, 1781 The Vermont Republic claimed what was called the "East Union," consisting of some towns in New Hampshire. They never gained control over the area.[6][7][8][9] Map of the international disputes involving the United States in central North America from April 4, 1781, to June 16, 1781 Map of the change to the international disputes involving the United States in central North America on April 4, 1781
June 16, 1781 The Vermont Republic claimed what was called the "West Union," consisting of some additional towns in New York. They never gained control over the area.[7][8][10][11] The specific date this occurred is unclear; sources suggest June 16, June 26, and July 18. Map of the international disputes involving the United States in central North America from June 16, 1781, to February 22, 1782 Map of the change to the international disputes involving the United States in central North America on June 16, 1781
February 22, 1782 The Vermont Republic abandoned its attempts to annex the East Union from New Hampshire, and the West Union from New York.[8][9][11][12] Map of the international disputes involving the United States in central North America from February 22, 1782, to May 12, 1784 Map of the change to the international disputes involving the United States in central North America on February 22, 1782
October 29, 1782 The federal government accepted the cession of February 17, 1780, from New York of its western claims; at their maximum interpretation, New York had claimed an area bordered by Lake Erie, Lake Huron and Lake Michigan, to the Illinois River, to the Mississippi River, to the Tennessee River and back north along the Appalachian Mountains, ending back at the border with Pennsylvania. It is unclear where this claim came from; many sources state that New York surrendered it, but very few elaborate on how it was obtained. One source states that it was a cession by the Six Nations, who had conquered much of the region.[13] However, these claims were never seriously enforced. It also ceded the western tip of the state, the Erie Triangle.[14][15] Map of the United States in central North America from October 29, 1782, to December 30, 1782 Map of the change to the United States in central North America on October 29, 1782
December 30, 1782 The Congress of the Confederation declared that the land that Connecticut has claimed in northeast Pennsylvania (and, unknown at the time, a small sliver of New York) was part of Pennsylvania, thus attempting to end the Pennamite–Yankee War.[16] While conflict would continue for some time, this was the end of formal claims by a state government. Map of the United States in central North America from December 30, 1782, to March 1, 1784 Map of the change to the United States in central North America on December 30, 1782
June 30, 1783 The Congress of the Confederation begins meeting in Princeton, thus making it the de facto capital.[2] no change to map
November 26, 1783 The Congress of the Confederation begins meeting in Annapolis, thus making it the de facto capital.[2] no change to map
March 1, 1784 Virginia ceded its western claims to the federal government;[17][18] Connecticut continued its western claims that overlapped Virginia's. Map of the United States in central North America from March 1, 1784, to July 13, 1787 Map of the change to the United States in central North America on March 1, 1784
May 12, 1784 Great Britain recognized the independence of the United States, ending their claim to the country.[19] Several areas in the north of the country had poorly defined borders with Great Britain, however.

The Peace of Paris also involved treaties with France and Spain; in particular, Great Britain ceded the Floridas to Spain. During Great Britain's ownership over West Florida, it had moved the border north, and the cession appeared to apply to the full extent of the British colony, which conflicted with the terms of the British-U.S. treaty. The local Spanish governors also made a move to occupy forts along the Mississippi River, with claims to everything south of the Tennessee River; it is unknown how official or strong these claims were, and as they conflict with the other Spanish claim involving the border of West Florida, they are not mapped.[20]

Map of the international disputes involving the United States in central North America from May 12, 1784, to March 4, 1791 Map of the change to the international disputes involving the United States in central North America on May 12, 1784
August 23, 1784 Representatives of several counties in western North Carolina, unhappy with the state's governance over the area, declared independence from the state as the State of Franklin.[21] The government of Franklin held some control over the area, and even petitioned for statehood, but would only last a few years. Map of the change to the United States in central North America on August 23, 1784
November 1, 1784 The Congress of the Confederation begins meeting in Trenton, thus making it the de facto capital.[2] no change to map
January 11, 1785 The Congress of the Confederation begins meeting in New York, thus making it the de facto capital.[2] no change to map
April 19, 1785 The federal government accepted the cession from Massachusetts of its extreme western claims, which consisted of a strip of land west Lake Huron and the Detroit River, bounded on the north by a line extending west from roughly halfway north through New Hampshire, and bounded on the south by a line extending from Massachusetts' southern border in its western half. However, these claims were never seriously enforced.[22] Map of the change to the United States in central North America on April 19, 1785
September 13, 1786 Connecticut surrendered most of its western claims to the federal government; it is unclear if they ever held control over the region.[23][24] Map of the change to the United States in central North America on September 13, 1786
December 16, 1786 Massachusetts surrendered its claim to western New York; it is unclear if Massachusetts ever held control over the region.[22] This land was later known as the Phelps and Gorham Purchase. Map of the change to the United States in central North America on December 16, 1786
July 13, 1787 The Northwest Ordinance organized the territory northwest of the Ohio River into the Northwest Territory.[25] Map of the United States in central North America from July 13, 1787, to April 2, 1790 Map of the change to the United States in central North America on July 13, 1787
August 9, 1787 South Carolina ceded its western claims to the federal government;[26] however, these were a result of incorrect surveying, and South Carolina never actually held claim to the land. This was unknown at the time, however, and the eastern part of this cession would be given to Georgia in 1802, despite Georgia technically already having claim to that land. Map of the change to the United States in central North America on August 9, 1787
December 7, 1787 Delaware became the 1st state to ratify the Constitution.[27] no change to map
December 12, 1787 Pennsylvania became the 2nd state to ratify the Constitution.[28] no change to map
December 18, 1787 New Jersey became the 3rd state to ratify the Constitution.[29] no change to map
January 2, 1788 Georgia became the 4th state to ratify the Constitution.[30] no change to map
January 6, 1788 Connecticut became the 5th state to ratify the Constitution.[31] no change to map
February 6, 1788 Massachusetts became the 6th state to ratify the Constitution.[32] no change to map
April 28, 1788 Maryland became the 7th state to ratify the Constitution.[33] no change to map
May 23, 1788 South Carolina became the 8th state to ratify the Constitution.[34] no change to map
June 21, 1788 New Hampshire became the 9th state to ratify the Constitution;[35] at this point, the Constitution became the active governing document of the country. no change to map
June 25, 1788 Virginia became the 10th state to ratify the Constitution.[36] no change to map
July 26, 1788 New York became the 11th state to ratify the Constitution.[37] no change to map
February 1789 John Sevier, governor of the State of Franklin, pledges allegiance to North Carolina, effectively ending Franklin's independence.[38] no change to map
August 7, 1789 The Northwest Territory was reorganized under the Constitution.[39] no change to map
November 21, 1789 North Carolina became the 12th state to ratify the Constitution.[40] no change to map
April 2, 1790 North Carolina ceded its western counties to the federal government.[41][42] Map of the United States in central North America from April 2, 1790, to May 26, 1790 Map of the change to the United States in central North America on April 2, 1790
May 26, 1790 The land ceded by North Carolina was organized as the Territory South of the River Ohio, commonly known as the Southwest Territory.[42][43] Map of the United States in central North America from May 26, 1790, to March 4, 1791 Map of the change to the United States in central North America on May 26, 1790
May 29, 1790 Rhode Island became the 13th state to ratify the Constitution.[44] no change to map
December 6, 1790 The United States Congress begins meeting in Philadelphia, thus making it the de facto capital.[2] no change to map
March 4, 1791 Vermont, which had been considered part of New York despite acting independently since 1777, was admitted as the 14th state. New York consented to Vermont's admission in 1790.[8][45] Map of the United States in central North America from March 4, 1791, to March 30, 1791 Map of the change to the United States in central North America on March 4, 1791
The Vermont Republic, by being admitted, no longer disputed any U.S. territory. Map of the international disputes involving the United States in central North America from March 4, 1791, to April 25, 1796 Map of the change to the international disputes involving the United States in central North America on March 4, 1791
March 30, 1791 The District of Columbia was formed from land ceded by Maryland and Virginia.[46][47] However, it did not yet have that name. At the time, it was simply referred to as the federal district. In September 1791, the commissioners in charge of planning the city would term it the "Territory of Columbia", and various laws refer to a District of Columbia, but sometimes informally. The area does not appear to have been formally named "District of Columbia" until at least the organic act of 1871.[48] However, since the name "Columbia" was used from very early on, and at least informally by the government, the map will use "District of Columbia" starting from this date. Map of the United States in central North America from March 30, 1791, to March 3, 1792 Map of the change to the United States in central North America on March 30, 1791
March 3, 1792 Pennsylvania purchased the Erie Triangle from the federal government.[49] Map of the United States in central North America from March 3, 1792, to June 1, 1792 Map of the change to the United States in central North America on March 3, 1792
June 1, 1792 The western counties of Virginia, which the state had agreed in 1789 to cede to the federal government,[50] were admitted as the 15th state, Kentucky.[51][52] Map of the United States in central North America from June 1, 1792, to June 1, 1796 Map of the change to the United States in central North America on June 1, 1792
February 29, 1796 Great Britain agreed to abandon several forts in the northwest, including Detroit, that it still occupied.[53] no change to map
April 25, 1796 The northern half of West Florida was ceded by Spain, resolving the dispute over the region.[54][55] Map of the international disputes involving the United States in central North America from April 25, 1796, to January 1, 1801 Map of the change to the international disputes involving the United States in central North America on April 25, 1796
June 1, 1796 Southwest Territory was admitted as the 16th state, Tennessee.[42][56] Map of the United States in central North America from June 1, 1796, to April 7, 1798 Map of the change to the United States in central North America on June 1, 1796
April 7, 1798 Due to the Yazoo Land Fraud, an act was signed by President John Adams authorizing him to appoint commissioners to negotiate with Georgia about ceding its western land. The act also created Mississippi Territory from the southwestern quarter of Georgia in the region recently ceded by West Florida, while maintaining that Georgia still held rights over the territory.[57][58] Map of the United States in central North America from April 7, 1798, to July 4, 1800 Map of the change to the United States in central North America on April 7, 1798
July 4, 1800 Indiana Territory was organized from the western bulk of Northwest Territory.[59][60] Map of the United States in central North America from July 4, 1800, to March 2, 1801 Map of the change to the United States in central North America on July 4, 1800
November 17, 1800 The United States Congress moves to Washington, D.C., now that it was ready to become the capital.[2] no change to map
January 1, 1801 The Kingdom of Great Britain united with the Kingdom of Ireland, renaming itself the United Kingdom.[61] Map of the international disputes involving the United States in central North America from January 1, 1801, to December 20, 1803 Map of the change to international disputes involving the United States in central North America on January 1, 1801
February 27, 1801 The District of Columbia was organized.[47][62] no change to map
March 2, 1801 Connecticut ceded its Western Reserve to the federal government, which assigned it to Northwest Territory.[63] The bill to do so was passed in the U.S. Congress on April 28, 1800, and it appears Connecticut approved it on June 9, 1800, but the presidential proclamation was not made until this date.[64] Map of the United States in central North America from March 2, 1801, to April 26, 1802 Map of the change to the United States in central North America on March 2, 1801
April 26, 1802 Georgia ceded its western half, known as the Yazoo Lands, to the federal government. At the same time, the federal government ceded to Georgia the eastern portion of the land previously ceded by South Carolina, though in reality Georgia technically already held title to the land, as the description of the earlier cession was based on an erroneous understanding of geography.[65] Map of the United States in central North America from April 26, 1802, to March 1, 1803 Map of the change to the United States in central North America on April 26, 1802
March 1, 1803 The southern half of Northwest Territory, along with a thin sliver of Indiana Territory, was admitted as the 17th state, Ohio. The remainder of Northwest Territory was transferred to Indiana Territory.[66][67] Map of the United States in central North America from March 1, 1803, to November 3, 1803 Map of the change to the United States in central North America on March 1, 1803
November 3, 1803 The border between Tennessee and Virginia was surveyed and established, ending the dispute over that part of the border. The border between Kentucky and Tennessee, despite following the same survey as this replaced, remained vaguely defined.[5][68] Map of the United States in central North America from November 3, 1803, to December 20, 1803 Map of the change to the United States in central North America on November 3, 1803
December 20, 1803 The United States purchased Louisiana from France. This is the date of the formal turnover in New Orleans; the purchase was completed on April 30, 1803.[69] The transfer would be recognized in St. Louis in Upper Louisiana on March 10, 1804, known as Three Flags Day. Map of the United States in central North America from December 20, 1803, to sometime in 1804 Map of the change to the United States in central North America on December 20, 1803
The extent of what constituted Louisiana was disputed with Spain, with the United States claiming the territory included much of West Florida, as well as a dispute over the southwestern border with New Spain. Map of the international disputes involving the United States in central North America from December 20, 1803, to September 26, 1810 Map of the change to the international disputes involving the United States in central North America on December 20, 1803
Sometime in 1804 The "Southwick Jog" was transferred from Connecticut to Massachusetts, to put to rest long-standing disagreements over the border between the two states.[22] Map of the United States in central North America from sometime in 1804 to March 27, 1804 Map of the change to the United States in central North America sometime in 1804
March 27, 1804 The land between Tennessee and Mississippi Territory previously ceded by Georgia was assigned to Mississippi Territory.[58][70] Map of the United States in central North America from March 27, 1804, to October 1, 1804 Map of the change to the United States in central North America on March 27, 1804
October 1, 1804 Orleans Territory was organized from the southern bit of the Louisiana Purchase, with the remainder being designated the District of Louisiana and placed under the jurisdiction of Indiana Territory.[71][72] Map of the United States in central North America from October 1, 1804, to June 30, 1805 Map of the change to the United States in central North America on October 1, 1804
June 30, 1805 Michigan Territory was organized from the northeastern quarter of Indiana Territory.[73][74] Michigan Territory disagreed with Ohio over the location of the border between them, beginning the dispute over the Toledo Strip, though at this point the dispute was technical rather than actual, since accurate mapping of the region was not yet available. Map of the United States in central North America from June 30, 1805, to July 4, 1805 Map of the change to the United States in central North America on June 30, 1805
July 4, 1805 The District of Louisiana was organized as Louisiana Territory.[72][75] Map of the United States in central North America from July 4, 1805, to March 1, 1809 Map of the change to the United States in central North America on July 4, 1805
March 1, 1809 Illinois Territory was organized from the western bulk of Indiana Territory.[76][77] Map of the United States in central North America from March 1, 1809, to April 30, 1812 Map of the change to the United States in central North America on March 1, 1809
September 26, 1810 The Republic of West Florida declared independence from Spain. It maintained some control over its territory.[78] Map of the international disputes involving the United States in central North America from September 23, 1810, to December 10, 1810 Map of the change to the international disputes involving the United States in central North America on September 26, 1810
December 10, 1810 Armed forces led by William C. C. Claiborne took possession of the portion of West Florida west of the Perdido River, following a proclamation on October 27, 1810, by President James Madison to do so. The United States had considered the region part of the Louisiana Purchase, and included the area which had revolted against Spanish Florida and formed the Republic of West Florida. Madison's proclamation stated that it was to be "taken as part" of Orleans Territory.[69][79][80] Map of the international disputes involving the United States in central North America from December 10, 1810, to August 16, 1812 Map of the change to the international disputes involving the United States in central North America on December 10, 1810
April 30, 1812 Most of Orleans Territory was admitted as the 18th state, Louisiana.[72][81] The remainder presumably became unorganized territory, as it had no definition for a while. Map of the United States in central North America from April 30, 1812, to May 14, 1812 Map of the change to the United States in central North America on April 30, 1812
May 14, 1812 The claimed portion of West Florida east of the Pearl River was assigned to Mississippi Territory, though at the time, the area around Mobile Bay remained under the control of Spanish Florida.[58][82] The U.S. at last militarily occupied Mobile and the surrounding area in April 1813. The land west of Mobile Bay to the Pearl River had been occupied and annexed de facto by the military in 1811.[83]:2a(map) Map of the United States in central North America from May 14, 1812, to June 4, 1812 Map of the change to the United States in central North America on May 14, 1812
June 4, 1812 Louisiana Territory was renamed Missouri Territory.[84][85] Map of the United States in central North America from June 4, 1812, to August 4, 1812 Map of the change to the United States in central North America on June 4, 1812
August 4, 1812 The remaining claimed portion of West Florida was added to Louisiana, following that state's assent to an act passed by the U.S. Congress on April 14, 1812.[86][87] Map of the United States in central North America from August 4, 1812, to December 11, 1816 Map of the change to the United States in central North America on August 4, 1812
August 16, 1812 The garrison at Fort Detroit surrenders, leading to the United Kingdom occupying Michigan Territory.[88] Map of the disputes involving the United States in central North America from August 16, 1812, to September 29, 1813 Map of the change to the disputes involving the United States in central North America on August 16, 1812
September 29, 1813 Fort Detroit was recaptured by American forces, regaining control over Michigan Territory.[88][89] Map of the disputes involving the United States in central North America from September 29, 1813, to February 22, 1821 Map of the change to the disputes involving the United States in central North America on September 29, 1813
August 24, 1814 British forces capture and burn Washington, D.C., but are forced to withdraw the next day. The functions of the capital were only momentarily suspended, though President James Madison took refuge in Brookville, Maryland.[90] no change to map
December 11, 1816 The southern part of Indiana Territory, along with small parts of Illinois Territory and Michigan Territory, were admitted as the 19th state, Indiana. A very small portion of Indiana Territory was transferred to Illinois Territory, and the northern part of Indiana Territory, across Lake Michigan, became unorganized territory.[60][91] Map of the United States in central North America from December 11, 1816, to March 3, 1817 Map of the change to the United States in central North America on December 11, 1816
March 3, 1817 Alabama Territory was organized from the eastern half of Mississippi Territory.[92][93] Map of the United States in central North America from March 3, 1817, to December 10, 1817 Map of the change to the United States in central North America on March 3, 1817
December 10, 1817 Mississippi Territory was admitted as the 20th state, Mississippi.[58][94] Map of the United States in central North America from December 10, 1817, to December 3, 1818 Map of the change to the United States in central North America on December 10, 1817
February 6, 1818 Alabama Territory created Tuskaloosa County with a description that inadvertently overlapped with Mississippi. It described the county's border as running "a due west course to, the Tombeckbe river; thence up the same to the Cotton Gin Port."[95] Unknown at the time, the origin of the Tombigbee River and Cotton Gin Port were in Mississippi. Map of the change to the United States in central North America on February 6, 1818
June 30, 1818 Per the terms of the Treaty of Ghent, the United Kingdom returns Moose Island to Maine, and the United States returns Campobello Island, Deer Island, and Grand Manan Island, all of which were captured from the other side during the War of 1812.[96] no change to map
December 3, 1818 The southern half of Illinois Territory was admitted as the 21st state, Illinois. The remainder of the territory, along with the unorganized territory that was recently part of Indiana Territory, were assigned to Michigan Territory.[77][97] Map of the United States in central North America from December 3, 1818, to January 30, 1819 Map of the change to the United States in central North America on December 3, 1818
January 30, 1819 The Treaty of 1818 went into effect, setting the 49th parallel north as the border with the United Kingdom west of the Lake of the Woods, and also establishing Oregon Country as a shared region with the United Kingdom.[98][99][100] Oregon Country had no defined northern limit, but it can be assumed that it did not encroach much upon Russian-held lands; this map uses the later established line at parallel 54°40′ north for simplicity. Map of the United States in northwest North America from January 30, 1819, to June 15, 1846

Map of the United States in central North America from January 30, 1819, to July 4, 1819

Map of the change to the United States in northwest North America on January 30, 1819

Map of the change to the United States in central North America on January 30, 1819

July 4, 1819 Arkansaw Territory was organized from the southern slice of Missouri Territory.[101][102] Map of the United States in central North America from July 4, 1819, to December 14, 1819 Map of the change to the United States in central North America on July 4, 1819
December 14, 1819 Alabama Territory was admitted as the 22nd state, Alabama.[93][103] The statehood act provided for a survey of the southern leg of the border with Mississippi, and to correct it if it was discovered the border encroaches upon Mississippi; it will later be discovered to do so. Map of the United States in central North America from December 14, 1819, to March 15, 1820 Map of the change to the United States in central North America on December 14, 1819
March 15, 1820 As part of the Missouri Compromise, the District of Maine, the northern and separate part of Massachusetts, was admitted as the 23rd state, Maine.[104][105] Map of the United States in central North America from March 15, 1820, to April 21, 1820 Map of the change to the United States in central North America on March 15, 1820
April 21, 1820 This is the first known date of the name "Arkansas Territory" being officially used instead of "Arkansaw Territory".[106] Map of the United States in central North America from April 21, 1820, to May 12, 1820 Map of the change to the United States in central North America on April 21, 1820
May 12, 1820 The border between Kentucky and Tennessee was established. To make up for the fact that the border between the Cumberland Gap and the Tennessee River veered north as much as 17 miles from 36°30′ north, a new survey was conducted starting at that point on the Mississippi River and moving east to the Tennessee River, hence granting Kentucky an addition in its southwest.[5] Map of the United States in central North America from May 12, 1820, to February 22, 1821 Map of the change to the United States in central North America on May 12, 1820
July 19, 1820 The overlap of the longitudinal southern border between Alabama and Mississippi was resolved, as per the act admitting Alabama as a state, because the provisional border encroached on Mississippi land.[107][108] As the result of a survey, the southern border terminus was moved about 3.8 miles to the east. The date when this happened is unclear; the sources available give either an unpublished report dated May 29, 1820, or the completion of the demarcation of the new line on July 19, 1820. Map of the change to the United States in central North Ameirca on July 19, 1820
December 19, 1820 Alabama redefined some county borders, ending its erroneous overlap of Mississippi.[109] Map of the change to the United States in central North America on December 19, 1820
February 22, 1821 The Adams–Onís Treaty took effect.[80] Many changes occurred:
  • The border with the holdings of Spain was concretely defined; previously, it had been the watershed of the Mississippi River and, for Oregon Country, the Columbia River. Now it followed specific rivers and parallels. The land exchanged in this fashion shouldn't really count as territory gained or lost, as was vaguely defined and weakly controlled.
  • The new border placed the "Neutral Ground" in Louisiana.
  • Florida was ceded to the United States; the formal handover would not happen until July.
  • Miller County had been created south of the Red River by Arkansas Territory, and was now on the Spanish side of the border, causing a new dispute. However, as this was a change made solely by the territory, and not confirmed by the federal government, in this map it is also considered a domestic dispute.
Map of the United States in central North America from February 22, 1821, to July 10, 1821 Map of the change to the United States in central North America on February 22, 1821
With West Florida being ceded, the dispute between it and Louisiana, Mississippi, and Alabama, was resolved. As noted above, Miller County was now disputed with Spain. Map of the international disputes involving the United States in central North America from February 22, 1821, to September 28, 1821 Map of the change to the international disputes involving the United States in central North America on February 22, 1821
July 10, 1821 East Florida was formally transferred to the United States by Spain.[110] Map of the United States in central North America from July 10, 1821, to July 17, 1821 Map of the change to the United States in central North America on July 10, 1821
July 17, 1821 West Florida was formally transferred to the United States by Spain.[110][111] Map of the United States in central North America from July 17, 1821, to August 10, 1821 Map of the change to the United States in central North America on July 17, 1821
August 10, 1821 The southeastern corner of Missouri Territory was admitted as the 24th state, Missouri. The rest became unorganized territory.[85][112] Map of the United States in central North America from August 10, 1821, to March 30, 1822 Map of the change to the United States in central North America on August 10, 1821
September 28, 1821 Mexico declared its independence from Spain, thus inheriting the dispute over Miller County, Arkansas Territory.[113] Map of the international disputes involving the United States in central North America from September 28, 1821, to July 9, 1832 Map of the change to the international disputes involving the United States in central North America on September 28, 1821
March 30, 1822 East Florida and West Florida were organized as Florida Territory.[114][115] Map of the United States in central North America from March 30, 1822, to May 26, 1824 Map of the change to the United States in central North America on March 30, 1822
May 26, 1824 The western half of Arkansas Territory was returned to unorganized territory.[102][116] Map of the United States in central North America from May 26, 1824, to May 6, 1828 Map of the change to the United States in central North America on May 26, 1824
January 12, 1825 A treaty with Russia established the parallel 54°40′ north as the northern border of Oregon Country for American purposes; a separate treaty created the same border between Russia and the United Kingdom.[117] As this was likely the de facto border anyway, the region is already mapped with this line. no change to map
May 6, 1828 A treaty with the Cherokee moved the western border of Arkansas Territory, returning part of it to unorganized territory.[102][118] Map of the United States in central North America from May 6, 1828, to June 28, 1834 Map of the change to the United States in central North America on May 6, 1828
July 9, 1832 The region of northern New Hampshire, which was disputed with the United Kingdom, declared independence as the Republic of Indian Stream.[119] While tiny, it does appear to have maintained some control over its territory. Map of the international disputes involving the United States in central North America from July 9, 1832, to August 5, 1835 Map of the change to the international disputes involving the United States in central North America on July 9, 1832
June 28, 1834 Michigan Territory gained a large parcel of land on its west from unorganized territory.[74][120] Map of the United States in central North America from June 28, 1834, to June 15, 1836 Map of the change to the United States in central North America on June 28, 1834
August 5, 1835 The Republic of Indian Stream recognized the jurisdiction of New Hampshire, thus ending their independence. The date given is of a communication sent to British authorities;[121] other sources note a resolution passed by the citizens of Indian Stream on April 2, 1836.[119] Map of the international disputes involving the United States in central North America from August 5, 1835, to March 2, 1836 Map of the change to the international disputes involving the United States in central North America on August 5, 1835
March 2, 1836 Texas declared independence from Mexico, thus joining the dispute with Arkansas Territory over Miller County.[122] Map of the international disputes involving the United States in central North America from March 2, 1836, to December 28, 1836 Map of the change to the international disputes involving the United States in central North America on March 2, 1836
June 15, 1836 Arkansas Territory was admitted as the 25th state, Arkansas.[102][123] Map of the United States in central North America from June 15, 1836, to July 3, 1836 Map of the change to the United States in central North America on June 15, 1836
July 3, 1836 Wisconsin Territory was organized from the western bulk of Michigan Territory.[124][125] Map of the United States in central North America from July 3, 1836, to December 14, 1836 Map of the change to the United States in central North America on July 3, 1836
December 14, 1836 Michigan Territory agreed to abandon its claim to the Toledo Strip, ending its dispute with Ohio.[126] Map of the United States in central North America from December 14, 1836, to January 26, 1837 Map of the change to the United States in central North America on December 14, 1836
December 28, 1836 Spain recognized the independence of Mexico, thus ending their involvement in the dispute over Miller County, Arkansas.[127] Map of the international disputes involving the United States in central North America from December 28, 1836, to May 21, 1840 Map of the change to the international disputes involving the United States in central North America on December 28, 1836
January 26, 1837 Michigan Territory was admitted as the 26th state, Michigan.[74][128] Map of the United States in central North America from January 26, 1837, to March 28, 1837 Map of the change to the United States in central North America on January 26, 1837
March 28, 1837 The Platte Purchase transferred some land from unorganized territory to northwest Missouri.[85][129] Map of the United States in central North America from March 28, 1837, to July 3, 1838 Map of the change to the United States in central North America on March 28, 1837
July 3, 1838 Iowa Territory was organized from the western half of Wisconsin Territory.[130][131] Map of the United States in central North America from July 3, 1838, to February 11, 1839 Map of the change to the United States in central North America on July 3, 1838
February 11, 1839 Missouri claimed an area north of its border in Iowa Territory, initiating the long dispute known as the Honey War.[132] Map of the United States in central North America from February 11, 1839, to May 21, 1840 Map of the change to the United States in central North America on February 11, 1839
May 21, 1840 Proper surveying was conducted along the border between Arkansas and Texas, and the area claimed by Arkansas for Miller County was held to not belong Arkansas.[133] Map of the United States in central North America from May 21, 1840, to November 10, 1842 Map of the change to the United States in central North America on May 21, 1840
With this decision, Miller County was no longer disputed with Mexico and Texas. Map of the international disputes involving the United States in central North America from May 21, 1840, to November 10, 1821 Map of the change to the international disputes involving the United States in central North America on May 21, 1840
November 10, 1842 The Webster-Ashburton Treaty defined the border with the United Kingdom east of the Rocky Mountains.[134][135] One source also mentions it very slightly altering the maritime boundary between Michigan and Wisconsin Territory, but it lacks details on the exact change.[136]

The border between New York and Vermont on the one side, and the United Kingdom on the other, was clarified by the treaty. In 1816, construction began on an unnamed fort nicknamed "Fort Blunder" on a peninsula in Lake Champlain that, while south of the surveyed border, was discovered to be north of the 45th parallel, which was the border set by the Treaty of Paris and thus in British territory. Consequently, construction on the fort was abandoned. The Webster-Ashburton Treaty specified that section of the border was to follow the surveyed line, rather than the exact parallel, thus moving the fort's area into the United States, and a new fort, Fort Montgomery, would be built on the spot in 1844.[137] As the earlier line was surveyed, even though it did not match the definition, it is considered the legitimate border.

Map of the United States in central North America from November 10, 1842, to March 3, 1845 Map of the change to the United States in central North America on November 10, 1842
The treaty resolved the disputes over the northern borders of Maine, Michigan, New Hampshire, and Wisconsin Territory. Map of the international disputes involving the United States in central North America from November 10, 1842, to December 29, 1845 Map of the change to the international disputes involving the United States in central North America on November 10, 1842
July 5, 1843 Local settlers created a provisional government for Oregon Country. While not official, it did maintain some jurisdiction over the area.[138] Map of the change to the United States in northwest North America on July 5, 1843Map of the change to the United States in central North America on July 5, 1843
March 3, 1845 Florida Territory was admitted as the 27th state, Florida.[115][139] Map of the United States in central North America from March 3, 1845, to December 29, 1845 Map of the change to the United States in central North America on March 3, 1845
December 29, 1845 The Republic of Texas was annexed and admitted as the 28th state, Texas.[140][141] Map of the United States in central North America from December 29, 1845, to June 15, 1846 Map of the change to the United States in central North America on December 29, 1845
The annexation of Texas brought with it a substantial territorial dispute with Mexico. While many sources state that Mexico recognized the independence of the eastern bulk of Texas, these treaties were rejected by the Mexican government. Map of the international disputes involving the United States in central North America from December 29, 1845, to June 15, 1846 Map of the change to the international disputes involving the United States in central North America on December 9, 1845
June 15, 1846 The Oregon Treaty established the 49th parallel north west of the Lake of the Woods as the continental border (so it did not include Vancouver Island) with land held by the United Kingdom. The sharing of Oregon Country ended, and the United States portion became unorganized territory.[142] Map of the United States in central North America from June 15, 1846, to December 28, 1846 Map of the change to the United States in northwest North America on June 15, 1846

Map of the change to the United States in central North America on June 15, 1846

The treaty was vague on which strait should be the border between Vancouver Island and the continent, thus causing a dispute over ownership of the San Juan Islands.[143] Map of the international disputes involving the United States in central North America from June 15, 1846, to July 4, 1848 Map of the change to the international disputes involving the United States in central North America on June 15, 1846
August 18, 1846 The U.S. Army of the West, led by Brigadier General Stephen W. Kearny, captured Santa Fe, the capital of Santa Fe de Nuevo México, Mexico, and proclaimed U.S. sovereignty over the territory.[144] no change to map
September 22, 1846 A code of laws known as the Kearny Code was created for the area of Santa Fe de Nuevo México that had been captured on August 18, 1846.[145][146] The region overlapped with Texas' claim, though Texas had little to no control over the area outside of its eastern quarter. Map of the change to the United States in central North America on September 22, 1846
December 28, 1846 The southern portion of Iowa Territory was admitted as the 29th state, Iowa. The remainder became unorganized territory.[131][147] Map of the United States in central North America from December 28, 1846, to March 13, 1847 Map of the change to the United States in central North America on December 28, 1846
March 13, 1847 The District of Columbia retroceded Alexandria County back to Virginia.[47] Congress passed the act on July 9, 1846,[148] residents of Alexandria County were proclaimed by the president to have agreed to it on September 7, 1846,[149] and Virginia took possession of the land on this date.[150] Map of the United States in central North America from March 13, 1847, to May 29, 1848 Map of the change to the United States in central North America on March 13, 1847
May 29, 1848 The southern bulk of Wisconsin Territory was admitted as the 30th state, Wisconsin. The remainder became unorganized territory.[125][151] However, the citizens of the remainder decided to continue on with a civil government, and even elected a delegate to the United States House of Representatives, whom would be seated on January 15, 1849, essentially making this region a de facto continuation of Wisconsin Territory.[152] Map of the United States in central North America from May 29, 1848, to July 4, 1848 Map of the change to the United States in central North America on May 29, 1848
July 4, 1848 A large parcel of land was ceded by Mexico following the Mexican-American War, consisting of its territories of Alta California and Santa Fe de Nuevo México, and its claim to Texas.[153] Map of the United States in central North America from July 4, 1848, to August 14, 1848 Map of the change to the United States in central North America on July 4, 1848
Due to a disagreement over the southern border of Santa Fe de Nuevo México, a border dispute begins.[154] Map of the international disputes involving the United States in central North America from July 4, 1848, to June 30, 1854 Map of the change to the international disputes involving the United States in central North America on July 4, 1848
August 14, 1848 Oregon Territory was organized from unorganized territory.[155][156] Map of the United States in central North America from August 14, 1848, to February 13, 1849 Map of the change to the United States in central North America on August 14, 1848
February 13, 1849 The boundary dispute between Iowa and Missouri known as the Honey War was resolved by the Supreme Court of the United States. The resulting border was the Sullivan Line that existed before the dispute, roughly splitting the two claims.[157] Map of the United States in central North America from February 13, 1849, to March 3, 1849 Map of the change to the United States in central North America on February 13, 1849
March 3, 1849 Minnesota Territory was organized from unorganized territory and the region that had been operating as de facto Wisconsin Territory.[158][159] Map of the United States in central North America from March 3, 1849, to September 9, 1850 Map of the change to the United States in central North America on March 3, 1849
March 12, 1849 A local government in the west formed the State of Deseret, and petitioned to be admitted to the United States. However, the proposal was rejected and in 1851, Utah Territory was formed instead.[160] The claimed area overlapped slightly with Texas' claimed area, as well as part of Oregon Territory. Map of the change to the United States in central North America on March 12, 1849
September 9, 1850 The western portion of the Mexican Cession was admitted as the 31st state, California.[161][162] The northern portion was organized as Utah Territory.[163][164] Part of Utah Territory overlapped with the portion of Texas that would be purchased on December 13, 1850, but the law authorizing such was passed on this day, so the borders of Utah Territory assumed the purchase will go through. Map of the United States in central North America from September 9, 1850, to December 13, 1850 Map of the change to the United States in central North America on September 9, 1850
December 9, 1850 The United Kingdom ceded less than one acre of underwater rock known as Horseshoe Reef in Lake Erie near Buffalo, New York for a lighthouse. It was surrounded by Canadian waters, thus creating a form of enclave.[165] Map of the United States in central North America from September 9, 1850, to December 13, 1850 Map of the change to the United States in central North America on December 9, 1850
December 13, 1850 The federal government purchased the western claims of Texas. New Mexico Territory was organized from the southern half of this land, along with some other territory in the Mexican Cession.[166][167] New Mexico Territory included all of the area that had been governed under the Kearny Code. Map of the United States in central North America from December 13, 1850, to March 2, 1853 Map of the change to the United States in central North America on December 13, 1850
April 5, 1851 The State of Deseret dissolved itself, its functions and territory largely having been superseded by Utah Territory.[168] no change to map
March 2, 1853 Washington Territory was organized from the northern half of Oregon Territory.[169][170] Map of the United States in central North America from March 2, 1853, to May 30, 1854 Map of the change to the United States in central North America on March 2, 1853
May 30, 1854 Kansas Territory and Nebraska Territory were organized from unorganized territory.[171][172][173] Much of the remaining unorganized territory became known as Indian Territory, designated as a place to resettle Indian tribes.

A small strip between Texas and Kansas Territory was unclaimed, due to falling south of Kansas Territory's border but north of the 36°30′ latitude established in the Missouri Compromise as the northern limit of slavery, and thus Texas could not have it. This became known as the Public Land Strip, or sometimes "No Man's Land."[174]

Map of the United States in central North America from May 30, 1854, to June 30, 1854 Map of the change to the United States in central North America on May 30, 1854
June 30, 1854 The United States purchased a large parcel from Mexico known as the Gadsden Purchase.[175] Map of the United States in central North America from June 30, 1854, to August 4, 1854 Map of the change to the United States in central North America on June 30, 1854
This resolved the border dispute, since the disputed land was included in the purchase.[154] Map of the international disputes involving the United States in central North America from June 30, 1854, to December 20, 1860 Map of the change to the international disputes involving the United States in central North America on June 30, 1854
August 4, 1854 The recently obtained Gadsden Purchase was assigned to New Mexico Territory.[167][176] Map of the United States in central North America from August 4, 1854, to May 11, 1858 Map of the change to the United States in central North America on August 4, 1854
January 11, 1855 Due to its remote location, Boston Corner was transferred from Massachusetts to New York.[177][178][179] Map of the United States in central North America from August 4, 1854, to May 11, 1858 Map of the change to the United States in central North America on January 11, 1855
October 28, 1856 Baker Island and Jarvis Island were claimed under the Guano Islands Act.[180] Map of the United States in the Pacific Ocean from October 28, 1856, to December 3, 1858 Map of the change to the United States in the Pacific Ocean on October 28, 1856
May 11, 1858 The eastern half of Minnesota Territory was admitted as the 32nd state, Minnesota. The remainder became unorganized territory.[159][181] Map of the United States in central North America from May 11, 1858, to February 14, 1859 Map of the change to the United States in central North America on May 11, 1858
August 31, 1858 Navassa Island was claimed under the Guano Islands Act.[180] Map of the United States in the Caribbean Sea from August 31, 1858, to December 30, 1862 Map of the change to the United States in the Caribbean Sea on August 31, 1858
December 3, 1858 Howland Island was claimed under the Guano Islands Act.[180] Map of the United States in the Pacific Ocean from December 3, 1858, to September 6, 1859 Map of the change to the United States in the Pacific Ocean on December 3, 1859
February 14, 1859 The western half of Oregon Territory was admitted as the 33rd state, Oregon. The remainder was transferred to Washington Territory.[156][182] Map of the United States in central North America from February 14, 1859, to February 8, 1860 Map of the change to the United States in central North America on February 14, 1859
July 6, 1859 A team of surveyors created the "Middleton Offset," a small notch in the border between Kentucky and Tennessee. It is unknown exactly why this was done.[183][184] Map of the United States in central North America from February 14, 1859, to February 8, 1860 Map of the change to the United States in central North America on July 6, 1859
September 6, 1859 Johnston Atoll was claimed under the Guano Islands Act.[180] It was claimed by Kingdom of Hawaii in 1858.[185] Map of the United States in the Pacific Ocean from September 6, 1859, to December 27, 1859 Map of the change to the United States in the Pacific Ocean on September 6, 1859
November 7, 1859 A local government was set up encompassing parts of the territories of Kansas, Nebraska, New Mexico, Utah, and Washington, with the name of Jefferson Territory. While never recognized by the federal government, it generally held control over the area until Colorado Territory was established, which adopted most of its laws.[186] Map of the change to the United States in central North America on November 7, 1859
December 27, 1859 Enderbury Island, McKean Island, Phoenix Island, and Starbuck Island were claimed under the Guano Islands Act.[180] Map of the United States in the Pacific Ocean from December 27, 1859, to December 29, 1859 Map of the change to the United States in the Pacific Ocean on December 27, 1859
December 29, 1859 Christmas Island and Malden Island were claimed under the Guano Islands Act.[180] Map of the United States in the Pacific Ocean from December 29, 1859, to February 8, 1860 Map of the change to the United States in the Pacific Ocean on December 29, 1859
February 8, 1860 Texas created Greer County, claiming part of Indian Territory based on a different understanding from the federal government of which river was the border between the two.[187] Map of the United States in central North America from February 8, 1860, to December 20, 1860 Map of the change to the United States in central North America on February 8, 1860
Atafu, Birnie Island, Butaritari, Caroline Island, Fanning Island, Flint Island, Gardner Island, Canton Island, Kingman Reef, Manihiki, Marakei, Nukunono, Palmyra Atoll, Penrhyn, Pukapuka, Rakahanga, Swains Island, Sydney Island, Vostok Island, and Washington Island were all claimed under the Guano Islands Act.[180] Many additional islands were listed as bonded on this date, but based on the coordinates they were either phantoms or duplicates. In addition, Sarah Ann Island was claimed, which may have existed and would be sighted as late as 1917, but has since disappeared.[188] Map of the United States in the Pacific Ocean from February 8, 1860, to April 15, 1862 Map of the change to the United States in the Pacific Ocean on February 8, 1860
December 20, 1860 South Carolina seceded from the Union.[189] Map of the international disputes involving the United States in central North America from December 20, 1860, to January 9, 1861 Map of the change to the international disputes involving the United States in central North America on December 20, 1860
In doing so, South Carolina withdrew from the United States Congress. Map of the United States in central North America from December 20, 1860, to January 9, 1861 Map of the change to the United States in central North America on December 20, 1860
January 9, 1861 Mississippi seceded from the Union.[189] Map of the international disputes involving the United States in central North America from January 9, 1861, to January 10, 1861 Map of the change to the international disputes involving the United States in central North America on January 9, 1861
In doing so, Mississippi withdrew from the United States Congress. Map of the United States in central North America from January 9, 1861, to January 10, 1861 Map of the change to the United States in central North America on January 9, 1861
January 10, 1861 Florida seceded from the Union.[189] Map of the international disputes involving the United States in central North America from January 10, 1861, to January 11, 1861 Map of the change to the international disputes involving the United States in central North America on January 10, 1861
In doing so, Florida withdrew from the United States Congress. Map of the United States in central North America from January 10, 1861, to January 11, 1861 Map of the change to the United States in central North America on January 10, 1861
January 11, 1861 Alabama seceded from the Union.[189] Map of the international disputes involving the United States in central North America from January 11, 1861, to January 19, 1861 Map of the change to the international disputes involving the United States in central North America on January 11, 1861
In doing so, Alabama withdrew from the United States Congress. Map of the United States in central North America from January 11, 1861, to January 19, 1861 Map of the change to the United States in central North America on January 11, 1861
January 19, 1861 Georgia seceded from the Union.[189] Map of the international disputes involving the United States in central North America from January 19, 1861, to January 26, 1861 Map of the change to the international disputes involving the United States in central North America on January 19, 1861
In doing so, Georgia withdrew from the United States Congress. Map of the United States in central North America from January 19, 1861, to January 26, 1861 Map of the change to the United States in central North America on January 19, 1861
January 26, 1861 Louisiana seceded from the Union.[189] Map of the international disputes involving the United States in central North America from January 26, 1861, to February 8, 1861 Map of the change to the international disputes involving the United States in central North America on January 26, 1861
In doing so, Louisiana withdrew from the United States Congress. However, districts 1 and 2 around New Orleans remained under Union control, and maintained representation in Congress. Map of the United States in central North America from January 26, 1861, to January 29, 1861 Map of the change to the United States in central North America on January 26, 1861
January 29, 1861 The eastern bulk of Kansas Territory was admitted as the 34th state, Kansas. The remainder became unorganized territory.[172][190] Map of the United States in central North America from January 29, 1861, to February 28, 1861 Map of the change to the United States in central North America on January 29, 1861
February 8, 1861 The Confederate States of America was formed by representatives of the seceded states of Alabama, Georgia, Florida, Louisiana, Mississippi, and South Carolina.[191] Map of the international disputes involving the United States in central North America from February 8, 1861, to March 2, 1861 Map of the change to the international disputes involving the United States in central North America on February 8, 1861
February 28, 1861 Colorado Territory was organized from portions of Nebraska Territory, New Mexico Territory, and Utah Territory, along with unorganized territory.[192][193] Map of the United States in central North America from February 28, 1861, to March 2, 1861 Map of the change to the United States in central North America on February 28, 1861
March 2, 1861 Texas seceded from the Union and was admitted to the Confederate States.[189][194] Map of the international disputes involving the United States in central North America from March 2, 1861, to March 28, 1861 Map of the change to the international disputes involving the United States in central North America on March 2, 1861
In doing so, Texas withdrew from the United States Congress.

Dakota Territory was organized from the northern bulk of Nebraska Territory and unorganized territory, and Nebraska Territory gained small portions of Utah Territory and Washington Territory.[173][195][196] Nevada Territory was organized from the western part of Utah Territory.[197][198]

Map of the United States in central North America from March 2, 1861, to April 17, 1861 Map of the change to the United States in central North America on March 2, 1861
March 28, 1861 Arizona Territory seceded from the Union.[199] Map of the international disputes involving the United States in central North America from March 28, 1861, to April 17, 1861 Map of the change to the international disputes involving the United States in central North America on March 28, 1861
April 17, 1861 Virginia seceded from the Union.[189] Map of the international disputes involving the United States in central North America from April 17, 1861, to May 6, 1861 Map of the change to the international disputes involving the United States in central North America on April 17, 1861
In doing so, Virginia withdrew from the United States Congress. However, districts 1 (along the Eastern Shore), 7 (near Washington, D.C.), 10, 11, and 12 (all three in the northwest of the state) remained under Union control, and maintained representation in Congress. Map of the United States in central North America from April 17, 1861, to May 6, 1861 Map of the change to the United States in central North America on April 17, 1861
May 6, 1861 Arkansas seceded from the Union.[189] Map of the international disputes involving the United States in central North America from May 6, 1861, to May 7, 1861 Map of the change to the international disputes involving the United States in central North America on May 6, 1861
In doing so, Arkansas withdrew from the United States Congress. Map of the United States in central North America from May 6, 1861, to May 20, 1861 Map of the change to the United States in central North America on May 6, 1861
May 7, 1861 Virginia was admitted to the Confederate States.[200] Map of the international disputes involving the United States in central North America from May 7, 1861, to May 20, 1861 Map of the change to the international disputes involving the United States in central North Americaon May 7, 1861
May 20, 1861 Arkansas was admitted to the Confederate States.[201]

North Carolina seceded from the Union.[189]

Map of the international disputes involving the United States in central North America from May 20, 1861, to June 8, 1861 Map of the change to the international disputes involving the United States in central North America on May 20, 1861
In doing so, North Carolina withdrew from the United States Congress. Map of the United States in central North America from May 20, 1861, to June 8, 1861 Map of the change to the United States in central North America on May 20, 1861
June 6, 1861 Robert Williamson Steele, governor of Jefferson Territory, declared the territory disbanded and handed over the government to the first governor of Colorado Territory.[186] no change to map
June 8, 1861 Tennessee seceded from the Union.[189] Map of the international disputes involving the United States in central North America from June 8, 1861, to July 2, 1861 Map of the change to the international disputes involving the United States in central North America on June 8, 1861
In doing so, Tennessee withdrew from the United States Congress. However, districts 2, 3, and 4 in the central part of the state remained under Union control, and maintained representation in Congress. Map of the United States in central North America from June 8, 1861, to March 1, 1862 Map of the change to the United States in central North America on June 8, 1861
July 2, 1861 Tennessee was admitted to the Confederate States.[202] Map of the international disputes involving the United States in central North America from July 2, 1861, to July 12, 1861 Map of the change to the international disputes involving the United States in central North America on July 2, 1861
July 12, 1861 Some of the Five Civilized Tribes signed various treaties with the Confederate States; on this date the Treaty with the Choctaw and Chickasaw was signed,[203] which granted them delegation in the Confederate Congress.[204] Due to the complex and split nature of Indian Territory during the American Civil War, this is when the list marks the region as generally allied with the Confederate States. Map of the international disputes involving the United States in central North America from July 12, 1861, to July 20, 1861 Map of the change to the international disputes involving the United States in central North America on July 12, 1861
July 20, 1861 North Carolina was admitted to the Confederate States. The law admitting the state required a presidential proclamation before it was to take effect,[205] which occurred on this date.[206] Map of the international disputes involving the United States in central North America from July 20, 1861, to August 1, 1861 Map of the change to the international disputes involving the United States in central North America on July 20, 1861
August 1, 1861 Following Confederate victory in the First Battle of Mesilla, Arizona Territory was proclaimed as part of the Confederate States.[207] Map of the international disputes involving the United States in central North America from August 1, 1861, to October 31, 1861 Map of the change to the international disputes involving the United States in central North America on August 1, 1861
October 31, 1861 A splinter government in Neosho, Missouri, declared the state's secession from the United States.[189] Map of the international disputes involving the United States in central North America from October 31, 1861, to November 20, 1861 Map of the change to the international disputes involving the United States in central North America on October 31, 1861
November 20, 1861 A splinter government in Bowling Green, Kentucky, declared the state's secession from the United States.[189] Map of the international disputes involving the United States in central North America from November 20, 1861, to November 28, 1861 Map of the change to the international disputes involving the United States in central North America on November 20, 1861
November 28, 1861 The splinter Neosho government of Missouri was admitted to the Confederate States. The Confederate States never held much power over the state, but it was given full representation in the legislature.[208] Map of the international disputes involving the United States in central North America from November 28, 1861, to December 10, 1861 Map of the change to the international disputes involving the United States in central North America on November 28, 1861
December 10, 1861 The splinter Bowling Green government of Kentucky was admitted to the Confederate States. The Confederate States never held much power over the state, but it was given full representation in the legislature.[209] Map of the international disputes involving the United States in central North America from December 10, 1861, to May 5, 1865 Map of the change to the international disputes involving the United States in central North America on December 10, 1861
March 1, 1862 A decree by the U.S. Supreme Court took effect, modifying the border between Massachusetts and Rhode Island.[22][210] Map of the United States in central North America from March 1, 1862, to July 14, 1862 Map of the change to the United States in central North America on March 1, 1862
April 15, 1862 Palmyra Atoll was annexed by Hawaii, and the U.S. claim falls dormant.[211] Map of the United States in the Pacific Ocean from April 15, 1862, to October 15, 1864 Map of the change to the United States in the Pacific Ocean on April 15, 1862
July 14, 1862 The western slice of Utah Territory was transferred to Nevada Territory, moving the border between the two east from the 39th meridian west of Washington to the 38th.[198][212] Map of the United States in central North America from July 14, 1862, to February 24, 1863 Map of the change to the United States in central North America on July 14, 1862
December 30, 1862 The Swan Islands were claimed under the Guano Islands Act.[180] Map of the United States in the Caribbean Sea from December 30, 1862, to December 11, 1868 Map of the change to the United States in the Caribbean Sea on December 30, 1862
February 24, 1863 Arizona Territory was organized from the western half of New Mexico Territory.[107][213] Map of the United States in central North America from February 24, 1863, to March 3, 1863 Map of the change to the United States in central North America on February 24, 1863
March 3, 1863 Idaho Territory was organized from the western portions of Dakota Territory and Nebraska Territory, and the eastern portion of Washington Territory.[214][215] Map of the United States in central North America from March 3, 1863, to March 4, 1863 Map of the change to the United States in central North America on March 3, 1863
March 4, 1863 Due to disruption in voting and low turnout, no one was allowed to take the seats held in the United States House of Representatives by the Unionist areas of Louisiana, Tennessee, and Virginia, effectively expelling the states.[citation needed] Map of the United States in central North America from March 4, 1863, to June 20, 1863 Map of the change to the United States in central North America on March 4, 1863
June 20, 1863 The northwestern counties of Virginia, represented by the Restored Government of Virginia in Wheeling, seceded from Virginia and were admitted to the Union as the 35th state, West Virginia.[216][217] Map of the United States in central North America from June 20, 1863, to August 5, 1863 Map of the change to the United States in central North America on June 20, 1863
August 5, 1863 Berkeley County was transferred by the federal government from Virginia to West Virginia;[218] Virginia, being a member of the Confederate States at the time, had no input into the matter. Map of the United States in central North America from August 5, 1863, to November 2, 1863 Map of the change to the United States in central North America on August 5, 1863
November 2, 1863 Jefferson County was transferred from Virginia to West Virginia.[219] Virginia, being a member of the Confederate States at the time, had no input into the matter. Map of the United States in central North America from November 2, 1863, to May 26, 1864 Map of the change to the United States in central North America on November 2, 1863
May 26, 1864 Montana Territory was organized from the northeast third of Idaho Territory.[220] The southeast third of Idaho Territory was transferred to Dakota Territory.[221] Map of the United States in central North America from May 26, 1864, to October 31, 1864 Map of the change to the United States in central North America on May 26, 1864
October 15, 1864 Malden Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from October 15, 1864, to December 26, 1866 Map of the change to the United States in the Pacific Ocean on October 15, 1864
October 31, 1864 Nevada Territory was admitted as the 36th state, Nevada.[198][223] Map of the United States in central North America from October 31, 1864, to May 5, 1866 Map of the change to the United States in central North America on October 31, 1864
May 5, 1865 The Confederate States cabinet met in Washington, Georgia, and dissolved.[224] Military surrenders were scattered throughout 1865, but the most important is regarded as that of the Army of Northern Virginia following the Battle of Appomattox Court House on April 9. Map of the international disputes involving the United States in central North America from May 5, 1865, to July 1, 1867 Map of the change to the international disputes involving the United States in central North America on May 5, 1865
May 5, 1866 The western slice of Utah Territory was transferred to Nevada, moving the border between the two east from the 38th meridian west of Washington to the 37th.[225] Map of the United States in central North America from May 5, 1866, to July 24, 1866 Map of the change to the United States in central North America on May 5, 1866
July 24, 1866 Tennessee was readmitted to the United States Congress.[226] Map of the United States in central North America from July 24, 1866, to January 18, 1867 Map of the change to the United States in central North America on July 24, 1866
December 26, 1866 Starbuck Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from December 26, 1866, to August 28, 1867 Map of the change to the United States in the Pacific Ocean on December 26, 1866
January 18, 1867 The northwestern corner of Arizona Territory was transferred to Nevada. The law transferring the land was approved May 5, 1866, but unlike the Utah Territory transfer of that day, this transfer was contingent on the state accepting it.[225][227] Map of the United States in central North America from January 18, 1867, to March 1, 1867 Map of the change to the United States in central North America on January 18, 1867
March 1, 1867 Nebraska Territory was admitted as the 37th state, Nebraska.[173][228] Map of the United States in central North America from March 1, 1867, to June 22, 1868 Map of the change to the United States in central North America on March 1, 1867
July 1, 1867 Canada was formed from several British colonies, including New Brunswick, thus inheriting the dispute over Machias Seal Island and North Rock. Map of the international disputes involving the United States in central North America from July 1, 1867, to July 20, 1871 Map of the change to the international disputes involving the United States in central North America on July 1, 1867
August 28, 1867 Midway Atoll was claimed.[229] The largest island of Midway, Sand Island, was claimed under the Guano Islands Act in 1858, but nothing more is known about this.[230] Map of the United States in the Pacific Ocean from August 28, 1867, to July 9, 1868 Map of the change to the United States in the Pacific Ocean on August 28, 1867
October 18, 1867 Alaska was purchased from Russia and designated the Department of Alaska.[231] Due to a lack of quality surveying, the southeastern border with British holdings was unclear and disputed.[232] Map of the United States in northwest North America from October 18, 1867, to July 15, 1870 Map of the change to the United States in northwest North America on October 18, 1867
June 22, 1868 Arkansas was readmitted to the United States Congress.[233] Map of the United States in central North America from June 22, 1868, to June 25, 1868 Map of the change to the United States in central North America on June 22, 1868
June 25, 1868 Florida was readmitted to the United States Congress.[234] Map of the United States in central North America from June 25, 1868, to July 4, 1868 Map of the change to the United States in central North America on June 25, 1868
July 4, 1868 North Carolina was readmitted to the United States Congress.[235] Map of the United States in central North America from July 4, 1868, to July 9, 1868 Map of the change to the United States in central North America on July 4, 1868
July 9, 1868 Louisiana and South Carolina were readmitted to the United States Congress.[236] Map of the United States in central North America from July 9, 1868, to July 13, 1868 Map of the change to the United States in central North America on July 9, 1868
Caroline Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from July 9, 1868, to sometime in 1873 Map of the change to the United States in the Pacific Ocean on July 9, 1868
July 13, 1868 Alabama was readmitted to the United States Congress.[237] Map of the United States in central North America from July 13, 1868, to July 25, 1868 Map of the change to the United States in central North America on July 13, 1868
July 25, 1868

Georgia was readmitted to the United States Congress.[238]

Wyoming Territory was organized from portions of Dakota, Idaho, and Utah Territories.[239][240] The territory would remain under the jurisdiction of Dakota Territory until its own government was organized on May 19, 1869.[241] The act organizing Wyoming Territory became law on this date, but it is unclear if the territory could be considered "organized" until May 19, 1869, as the act specifies it was not to take effect until a government is organized; however, all sources use this date as the creation, and most use it for the organization, of the territory. A tiny portion of Dakota Territory was erroneously left behind on the western side of Wyoming Territory.[242]

Map of the United States in central North America from July 25, 1868, to March 3, 1869 Map of the change to the United States in central North America on July 25, 1868
August 12, 1868 The list of bonded guano island claims mentions "Islands in Caribbean Sea not named" bonded on this date, but it is unknown what this is referring to.[180] no change to map
December 11, 1868 Serrana Bank was claimed under the Guano Islands Act.[180] Colombia has claimed it throughout its history. Map of the United States in the Caribbean Sea from December 11, 1868, to November 22, 1869 Map of the change to the United States in the Caribbean Sea on December 11, 1868
March 3, 1869 Georgia was again expelled from the U.S. Congress following failures of Reconstruction in the state.[243] Map of the United States in central North America from March 3, 1869, to January 26, 1870 Map of the change to the United States in central North America on March 3, 1869
November 22, 1869 Bajo Nuevo Bank, Pedro Cays, Quita Sueño Bank, and Roncador Bank were claimed under the Guano Islands Act.[180][229] Except for Pedro Cays, Colombia has claimed them throughout its history. Map of the United States in the Caribbean Sea from November 22, 1869, to September 8, 1879 Map of the change to the United States in the Caribbean Sea on November 22, 1869
January 26, 1870 Virginia was readmitted to the United States Congress.[244] Map of the United States in central North America from January 26, 1870, to February 26, 1870 Map of the change to the United States in central North America on January 26, 1870
February 23, 1870 Mississippi was readmitted to the United States Congress.[245] Map of the United States in central North America from February 23, 1870, to March 30, 1870 Map of the change to the United States in central North America on February 23, 1870
March 30, 1870 Texas was readmitted to the United States Congress.[246] Map of the United States in central North America from March 30, 1870, to July 15, 1870 Map of the change to the United States in central North America on March 30, 1870
July 15, 1870 Georgia was again readmitted to the United States Congress.[247] Map of the United States in central North America from July 15, 1870, to February 17, 1873 Map of the change to the United States in central North America on July 15, 1870
The North-Western Territory was transferred by the United Kingdom to Canada, thus transferring its portion of the Alaska boundary dispute.[248] Map of the United States in northwest North America from July 15, 1870, to July 20, 1871 Map of the change to the United States in northwest North America on July 15, 1870
February 9, 1871 A small parcel was transferred from Dakota Territory to Nebraska following a sudden change in course of the Missouri River.[173][249] Map of the United States in central North America from July 15, 1870, to February 17, 1873 Map of the change to the United States in central North America on February 9, 1871
July 20, 1871 British Columbia joined Canada, transferring the dispute over the San Juan Islands as well as its portion of the Alaska boundary dispute.[250] Map of the United States in northwest North America from July 20, 1871, to May 17, 1884

Map of the international disputes involving the United States in central North America from July 20, 1871, to October 21, 1872

Map of the change to the United States in northwest North America on July 20, 1871

Map of the change to the international disputes involving the United States in central North America on July 20, 1871

October 21, 1872 The dispute with Canada over the San Juan Islands was resolved in the favor of the United States claim.[143] Map of the international disputes involving the United States in central North America from October 21, 1872, to the present Map of the change to the international disputes involving the United States in central North America on October 21, 1872
Sometime in 1873 Vostok Island was claimed by the United Kingdom.[251] Map of the United States in the Pacific Ocean from sometime in 1873 to August 13, 1877 Map of the change to the United States in the Pacific Ocean sometime in 1873
February 17, 1873 The small portion of Dakota Territory that was left behind due to an erroneous description of the territory's borders was transferred to Montana Territory.[242][252] Map of the United States in Central North America from February 17, 1873, to August 1, 1876 Map of the change to the United States in Central North America on February 17, 1873
August 1, 1876 Colorado Territory was admitted as the 38th state, Colorado.[193][253] Map of the United States in central North America from August 1, 1876, to May 23, 1882 Map of the change to the United States in central North America on August 1, 1876
August 13, 1877 The United Kingdom created the British Western Pacific Territories, including all islands thought previously unclaimed. It included Atafu and Nukunono Atolls.[254] Map of the United States in the Pacific Ocean from August 13, 1877, to sometime in 1881 Map of the change to the United States in the Pacific Ocean on August 13, 1877
March 3, 1879 The border across the Chesapeake Bay between Maryland and Virginia was decided via arbitration. The new border cuts across Smith Island, likely meaning that the southern portion of the island was transferred to Virginia, but due to the general lack of concrete borders in the area for so long due to surveying errors, it is unknown if any land actually changed hands.[255] too vague to map
September 8, 1879 Arenas Key, claimed by Mexico, and Serranilla Bank, claimed by Colombia, were claimed by the U.S. under the Guano Islands Act;[180] according to the Office of Insular Affairs, Serranilla Bank was claimed again on September 13, 1880.[229] Map of the United States in the Caribbean Sea from September 8, 1879, to September 13, 1880 Map of the change to the United States in the Caribbean Sea on September 8, 1879
April 7, 1880 A very small area of Vermont near Fair Haven was transferred to New York due to a change in the course in the Poultney River.[8][256] The specific area was very small and poorly documented, so it is not mapped. too small to map
September 13, 1880 Western Triangle Island, claimed by Mexico, was claimed by the U.S. under the Guano Islands Act.[180] The list of bonded claims also mentions a "De Anes" island claimed on this date, with coordinates matching Isla de Aves; however, the same list points out that the claim to "Aves Island" was found to be invalid. Map of the United States in the Caribbean Sea from September 13, 1880, to June 1, 1882 Map of the change to the United States in the Caribbean Sea on September 13, 1880
Sometime in 1881 Flint Island was claimed by the United Kingdom.[257] Map of the United States in the Pacific Ocean from sometime in 1881 to March 15, 1888 Map of the change to the United States in the Pacific Ocean sometime in 1881
May 23, 1882 The area between the 43rd parallel north and the Keya Paha and Niobrara Rivers was transferred from Dakota Territory to Nebraska.[196][258] The bill was passed in the U.S. Congress on March 28, and accepted by the Nebraska legislature on May 23.[259] Map of the United States in central North America from May 23, 1882, to November 2, 1889 Map of the change to the United States in central North America on May 23, 1882
June 1, 1882 The Morant Cays and Pedro Cays were annexed by the United Kingdom to Jamaica; it appears they were no longer claimed by the U.S. after this.[260] Map of the United States in the Caribbean Sea from June 1, 1882, to April 11, 1899 Map of the change to the United States in the Caribbean Sea on June 1, 1882
May 17, 1884 The Department of Alaska was organized into the District of Alaska.[261] Map of the United States in northwest North America from May 17, 1884, to October 20, 1903 Map of the change to the United States in northwest North America on May 17, 1884
June 21, 1884 The Alacrans Islands, claimed by Mexico, were claimed under the Guano Islands Act.[180] Map of the United States in the Caribbean Sea from June 21, 1884, to November 17, 1894 Map of the change to the United States in the Caribbean Sea on June 21, 1884
March 15, 1888 Fanning Island was annexed by the United Kingdom; it appears the island was no longer claimed by the U.S. after this.[262] Map of the United States in the Pacific Ocean from March 15, 1888, to March 17, 1888 Map of the change to the United States in the Pacific Ocean on March 15, 1888
March 17, 1888 Christmas Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from March 17, 1888, to October 26, 1888 Map of the change to the United States in the Pacific Ocean on March 17, 1888
October 26, 1888 The Cook Islands became a protectorate of the United Kingdom, thus initiating a claim on the atolls of Pukapuka, Manihiki, Penrhyn, and Rakahanga.[222] Map of the United States in the Pacific Ocean from October 26, 1888, to May 29, 1889 Map of the change to the United States in the Pacific Ocean on October 26, 1888
May 29, 1889 Washington Island was annexed by the United Kingdom; it appears the island was no longer claimed by the U.S. after this.[262] Map of the United States in the Pacific Ocean from May 29, 1889, to June 3, 1889 Map of the change to the United States in the Pacific Ocean on May 29, 1889
June 3, 1889 Jarvis Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from June 3, 1889, to June 26, 1889 Map of the change to the United States in the Pacific Ocean on June 3, 1889
June 26, 1889 Sydney Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from June 26, 1889, to June 29, 1889 Map of the change to the United States in the Pacific Ocean on June 26, 1889
June 29, 1889 Phoenix Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from June 29, 1889, to July 10, 1889 Map of the change to the United States in the Pacific Ocean on June 29, 1889
July 10, 1889 Birnie Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from July 10, 1889, to March 8, 1892 Map of the change to the United States in the Pacific Ocean on July 10, 1889
November 2, 1889 Dakota Territory was split and admitted as the 39th state, North Dakota, and the 40th state, South Dakota.[196][263] Map of the United States in central North America from November 2, 1889, to November 8, 1889 Map of the change to the United States in central North America on November 2, 1889
November 8, 1889 Montana Territory was admitted as the 41st state, Montana.[220][263] Map of the United States in central North America from November 8, 1889, to November 11, 1889 Map of the change to the United States in central North America on November 8, 1889
November 11, 1889 Washington Territory was admitted as the 42nd state, Washington.[170][263] Map of the United States in central North America from November 11, 1889, to May 2, 1890 Map of the change to the United States in central North America on November 11, 1889
May 2, 1890 Oklahoma Territory was organized from the Public Land Strip and the western half of Indian Territory, except for the Cherokee Outlet which could be added later upon cession from the Cherokee.[264][265] Map of the United States in central North America from May 2, 1890, to July 3, 1890 Map of the change to the United States in central North America on May 2, 1890
July 3, 1890 Idaho Territory was admitted as the 43rd state, Idaho.[215][266] Map of the United States in central North America from July 3, 1890, to July 10, 1890 Map of the change to the United States in central North America on July 3, 1890
July 10, 1890 Wyoming Territory was admitted as the 44th state, Wyoming.[240][267] Map of the United States in central North America from July 10, 1890, to September 16, 1893 Map of the change to the United States in central North America on July 10, 1890
March 8, 1892 The Gilbert Islands became a protectorate of the United Kingdom, thus initiating a claim on Butaritari and Marakei.[222] No record of a U.S. claim exists after this point, so it is assumed this is when the claim fell dormant. Map of the United States in the Pacific Ocean from March 8, 1892, to May 28, 1892 Map of the change to the United States in the Pacific Ocean on March 8, 1892
May 28, 1892 Gardner Island was claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from May 28, 1892, to August 12, 1898 Map of the change to the United States in the Pacific Ocean on May 28, 1892
September 16, 1893 Per a treaty with the Cherokee, the federal government purchased the Cherokee Outlet and opened it to settlement, transferring it from Indian Territory to Oklahoma Territory as provided in the Oklahoma Organic Act.[265][268] Map of the United States in central North America from September 16, 1893, to January 4, 1896 Map of the change to the United States in central North America on September 16, 1893
November 17, 1894 The Alacrans Islands, Arenas Key, and Western Triangle Island were stricken from the list of claimed guano islands.[180] Map of the United States in the Caribbean Sea from November 17, 1894, to April 11, 1899 Map of the change to the United States in the Caribbean Sea on November 17, 1894
January 4, 1896 Utah Territory was admitted as the 45th state, Utah.[164][269] Map of the United States in central North America from January 4, 1896, to March 16, 1896 Map of the change to the United States in central North America on January 4, 1896
March 16, 1896 The dispute between the federal government, on behalf of Oklahoma Territory, and Texas over Greer County was resolved in favor of the federal claim.[187] Map of the United States in central North America from March 16, 1896, to November 16, 1907 Map of the change to the United States in central North America on March 16, 1896
July 24, 1897 Due to an earlier shift in the course of the Missouri River, an island was transferred from Nebraska to South Dakota.[270] Map of the United States in central North America from March 16, 1896, to November 16, 1907 Map of the change to the United States in central North America on July 24, 1897
August 12, 1898 The Republic of Hawaii was annexed.[271] The ceremony to transfer sovereignty occurred on this date; the bill was signed on July 7, 1898.[272] Johnston Atoll was not included with Hawaii. Map of the United States in the Pacific Ocean from August 12, 1898, to January 17, 1899 Map of the change to the United States in the Pacific Ocean on August 12, 1898
January 17, 1899 Wake Island was claimed.[273] Map of the United States in the Pacific Ocean from January 17, 1899, to April 11, 1899 Map of the change to the United States in the Pacific Ocean on January 17, 1899
April 11, 1899 Guam, Porto Rico, and, after a payment of $20 million, the Philippines were ceded by Spain following the Spanish–American War.[274] The ceded region for the Philippines included the island of Palmas, which was administered by the Netherlands in the Dutch East Indies. This overlap would not be noticed until January 21, 1906.[275] Map of the United States in the Pacific Ocean from April 11, 1899, to February 16, 1900

Map of the United States in the Caribbean Sea from April 11, 1899, to May 4, 1904

Map of the change to the United States in the Pacific Ocean on April 11, 1899

Map of the change to the United States in the Caribbean Sea on April 11, 1899

February 16, 1900 The United States took ownership of the Samoan Islands east of the 171st meridian west, per the terms of the Tripartite Convention.[276] Map of the United States in the Pacific Ocean from February 16, 1900, to February 19, 1900 Map of the change to the United States in the Pacific Ocean on February 16, 1900
February 19, 1900 The newly acquired Samoan islands were established as Naval Station, Tutuila. It included all of the islands granted by the Tripartite Convention, though formal cession of the islands by local authorities would take place later in 1900 and 1904.[276] Map of the United States in the Pacific Ocean from February 19, 1900, to June 14, 1900 Map of the change to the United States in the Pacific Ocean on February 19, 1900
April 10, 1900 The island of Tutuila was formally ceded to the United States and added to Naval Station, Tutuila.[276] As the United States had already claimed the island on February 19, 1900, no change is mapped. The treaty would be ratified by the U.S. Congress on February 20, 1929. no change to map
April 12, 1900 Porto Rico was organized into a civil territory.[277] no change to map
June 14, 1900 The former Republic of Hawaii was organized into Hawaii Territory.[278] Map of the United States in the Pacific Ocean from June 14, 1900, to March 23, 1901 Map of the change to the United States in the Pacific Ocean on June 14, 1900
March 3, 1901 The transfer for a thin sliver of Bristol, Tennessee, to Bristol, Virginia, was approved by the U.S. Congress after having been approved by both states.[279][280] The location of the border along Main Street (now State Street) between the two cities was either the northern sidewalk of the street, or down the middle of the street; Tennessee's cession of the northern half of the street laid the issue to rest. too small to map
March 23, 1901 The islands of Cagayan de Sulu and Sibutu, and their associated islands, were purchased from Spain and assigned to the Philippines. The borders specified in the Treaty of Paris of 1898 had excluded these islands; the new treaty simply ceded "any and all islands belonging to the Philippine Archipelago".[281][282] Map of the United States in the Pacific Ocean from March 23, 1901, to July 17, 1911 Map of the change to the United States in the Pacific Ocean on March 23, 1901
February 23, 1903 Land along southern Guantánamo Bay was leased in perpetuity from Cuba for use as a naval base.[283] no change to map
October 20, 1903 The Alaska boundary dispute with Canada was resolved, generally in the favor of the United States claim.[232] However, it created a new dispute over the waters of the Dixon Entrance, which remain disputed until the present day. Map of the United States in northwest North America from October 20, 1903, to August 24, 1912 Map of the change to the United States in northwest North America on October 20, 1903
May 4, 1904 The United States took ownership of the Panama Canal Zone. At this stage, only the most basic borders were defined; it was a zone surrounding the canal on each side for five miles, but excluded the cities of Colón and Panama City, which remained exclaves of Panama, as well as the water for their harbors.[284] The Hay–Bunau-Varilla Treaty granting it to the United States was ratified on February 26, 1904.[285] A formal border agreement, which also gave the Canal Zone some land and a lighthouse in northwest Colón, would be ratified on June 15, 1904.[286][287] Map of the United States in the Caribbean Sea from May 4, 1904, to May 1, 1915 Map of the change to the United States in the Caribbean Sea on May 4, 1904
July 16, 1904 The Manuʻa islands were formally ceded to the United States and added to Naval Station, Tutuila.[276] As the United States had already claimed the islands on February 19, 1900, no change is mapped. The treaty would be ratified by the U.S. Congress on February 20, 1929. no change to map
December 12, 1904 The "Taft Agreement" was made with Panama on December 3, with one of its sections refining the maritime boundary of the harbor of Panama City and the Panama Canal Zone.[288][289] It became effective December 12. no change to map
February 10, 1905 The border between Arkansas and Indian Territory was slightly adjusted near Fort Smith, Arkansas, transferring a small amount of land to Arkansas.[290][291] Map of the United States in central North America from March 16, 1896, to November 16, 1907 Map of the change to the United States in central North America on February 10, 1905
November 16, 1907 Indian Territory and Oklahoma Territory were combined and admitted as the 46th state, Oklahoma.[265][292] Map of the United States in central North America from November 16, 1907, to January 1, 1909 Map of the change to the United States in central North America on November 16, 1907
April 11, 1908 A boundary treaty with the United Kingdom on behalf of Canada redefined the maritime borders between the United States and Canada.[293] Among other changes, this "de-enclaved" Horseshoe Reef in Lake Erie by making the water around it contiguous with the water on the American side of the border.[165][294] no change to map
January 1, 1909 The new Constitution of Michigan included some area of Wisconsin within its definition of Michigan.[295] Map of the United States in central North America from January 1, 1909, to January 6, 1912 Map of the change to the United States in central North America on January 1, 1909
August 20, 1910 A boundary treaty with the United Kingdom on behalf of Canada addressed a slight uncertainty in the maritime border in Passamaquoddy Bay between Maine and Canada.[296][297] The border was adjusted to run east of Pope's Folly Island, which previously lay on the border line, and had been the subject of some debate for many years.[298][299] Map of the United States in central North America from January 1, 1909, to January 6, 1912 Map of the change to the United States in central North America on August 20, 1910
July 17, 1911 Naval Station, Tutuila, was renamed American Samoa;[300] the station continued to operate separate from territorial governance until 1951. Map of the United States in the Pacific Ocean from July 17, 1911, to March 4, 1925 Map of the change to the United States in the Pacific Ocean on July 17, 1911
January 6, 1912 New Mexico Territory was admitted as the 47th state, New Mexico.[167][301] Map of the United States in central North America from January 6, 1912, to February 14, 1912 Map of the change to the United States in central North America on January 6, 1912
February 14, 1912 Arizona Territory was admitted as the 48th state, Arizona.[107][302] Map of the United States in central North America from February 14, 1912, to January 31, 1913 Map of the change to the United States in central North America on February 14, 1912
August 24, 1912 The District of Alaska was reorganized as the Alaska Territory.[303] Map of the United States in northwest North America from August 24, 1912, to January 3, 1959 Map of the change to the United States in northwest North America on August 24, 1912
January 31, 1913 New Mexico filed suit in the U.S. Supreme Court against Texas over the "Country Club Dispute".[304] Map of the United States in central North America from January 31, 1913, to June 30, 1921 Map of the change to the United States in central North America on January 31, 1913
August 5, 1914 The Corn Islands were leased from Nicaragua for a period of 99 years; however, this was not a full transfer of sovereignty, and the islands were never administered as an insular area.[305] no change to map
May 1, 1915 The borders of the Panama Canal Zone were explicitly defined. Whereas the original definition was a simple corridor surrounding the canal, this treaty specified the actual border. Among the changes this caused were: a slice of Canal Zone was ceded to Panama so Panama City would be connected with the rest of the country; the middle island of the Las Tres Hermanas Islands, which marked the border of Panama City's harbor, was wholly made part of Canal Zone; Gatun Lake and other surrounding waters were formally annexed to the territory; and an area of northwest Colón was ceded to Canal Zone.[306][307][308] Map of the United States in the Caribbean Sea from May 1, 1915, to March 31, 1917 Map of the change to the United States in the Caribbean Sea on May 1, 1915
May 27, 1915 Under Article II of the 1903 Treaty, the U.S. expropriated from Panama several areas around the mouth of the Rio Chagres, annexing them to the Panama Canal Zone.[309] too small to map
December 8, 1915 The U.S. expropriated from Panama a triangle of land, which included the historic Fort San Lorenzo, between the Rio Chagres, Caribbean Sea and the Panama Canal Zone, to which it was annexed.[309] too small to map
January 17, 1916 Navassa Island was formally claimed for lighthouse purposes.[310] no change to map
March 31, 1917 The Danish Virgin Islands were purchased from Denmark[311] and renamed the United States Virgin Islands.[312] Map of the United States in the Caribbean Sea from March 31, 1917, to June 5, 1924 Map of the change to the United States in the Caribbean Sea on March 31, 1917
July 12, 1918 The U.S. expropriated from Panama 2.6 hectares of land at Punta Paitilla in Panama City and annexed it to the Panama Canal Zone.[313] That area was enlarged to about 50 hectares within several months.[309][314][315][316] too small to map
August 21, 1918 The U.S. expropriated from Panama 1282 hectares of land between the Rio Chagres and the Quebrada Majagual and annexed it to the Panama Canal Zone.[309][316] too small to map
September 13, 1918 Minnesota and Wisconsin exchanged islands in the Mississippi River: Island Seventytwo was transferred from Wisconsin to the Minnesota town of Winona, and Barron's Island was transferred from Minnesota to the Wisconsin town of La Crosse.[125][317] Map of the United States in central North America from January 31, 1913, to June 30, 1921 Map of the change to the United States in central North America on September 13, 1918
September 18, 1919 The island of Largo Remo is annexed to the Panama Canal Zone under the U.S. right of expropriation in the 1903 Canal Treaty.[318] Map of the United States in the Caribbean Sea from March 31, 1917, to June 5, 1924 Map of the change to the United States in the Caribbean Sea on September 18, 1919, to November 15, 1923
June 16, 1920 Fifteen hectares on the island of Taboga Island are annexed to the Panama Canal Zone.[319] too small to map
June 30, 1921 The "Wedge" dispute between Delaware and Pennsylvania was resolved in Delaware's favor. The disputed land had generally been administered by Delaware, even electing a member of the Delaware legislature in the mid-19th century,[320] but federal maps had included the land as part of Pennsylvania at least as late as 1900.[321] The states had agreed on a resolution, and it was affirmed by an act of the U.S. Congress on this date.[322][323] Some sources, contemporary and modern, note that, in the original process of resurveying the border in 1892, a very thin, horn-shaped region along the arc was transferred from Delaware to Pennsylvania;[320][323][324] however, no federal maps found reflect this, and it is unclear if this transfer actually occurred. Map of the United States in central North America from June 30, 1921, to October 8, 1923 Map of the change to the United States in central North America on June 30, 1921
May 10, 1922 Kingman Reef was formally claimed.[325] no change to map
October 8, 1923 Michigan expanded its claim to Wisconsin territory, though Wisconsin never lost control over the area.[295] Map of the United States in central North America from October 8, 1923, to March 1, 1926 Map of the change to the United States in central North America on October 8, 1923
November 15, 1923 The Swan Islands were claimed by Honduras.[326] Map of the United States in the Caribbean Sea from November 15, 1923, to June 5, 1924 Map of the change to the United States in the Caribbean Sea on November 15, 1923
June 5, 1924 The future area for Madden Lake was annexed to the Panama Canal Zone under the U.S. right of expropriation in the 1903 Canal Treaty.[287][327] Map of the United States in the Caribbean Sea from June 5, 1924, to March 17, 1932 Map of the change to the United States in the Caribbean Sea on June 5, 1924
March 4, 1925 Swains Island was added to American Samoa.[328] Map of the United States in the Pacific Ocean from March 4, 1925, to April 4, 1928 Map of the change to the United States in the Pacific Ocean on March 4, 1925
July 17, 1925 The border with Canada was adjusted in several places.[329][330] The only change to a land border redefined how the border between the Lake of the Woods and the Rocky Mountains should be considered; previously, the border followed the curve of the parallel between each border monument, while the treaty changed this to straight lines between each monument. Through this, the United States netted a gain of between 30 and 35 acres of land. Due to the extremely small shift, the lack of specific documentation of where the changes occurred, and the lack of any human impact, this change is not mapped. There was also a change to the border in the Lake of the Woods; due to inaccurate surveying, the previous border intersected itself several times in the lake, creating enclaves of United States water surrounded by Canadian water. The treaty changed the border to use the southernmost intersection as the northwestern point of the Lake of the Woods. Finally, the maritime border in the Bay of Fundy was adjusted, netting Canada roughly 9 acres of water. too small to map
March 1, 1926 The Supreme Court of the United States resolved the conflict between Michigan and Wisconsin in the favor of Wisconsin.[295] Map of the United States in central North America from March 1, 1926, to November 22, 1926 Map of the change to the United States in central North America on March 1, 1926
July 29, 1926 Johnston Atoll was established as a federal bird refuge and placed under the jurisdiction of the U.S. Department of Agriculture.[331] The atoll had originally been claimed by both the U.S. and Hawaii in in 1858, but little activity apart from guano mining had taken place, and it had been largely abandoned for decades.[185] no change to map
November 22, 1926 The Supreme Court of the United States defined the border between Michigan and Wisconsin, transferring all islands south of the Quinnesec Falls on the Menominee River to Wisconsin, and all islands north of the falls to Michigan; it is unknown specifically which islands were transferred in this fashion. However, an error in the border description introduced a small overlap between the two states over several islands in Lake Michigan north of the Door Peninsula.[332] Map of the United States in central North America from November 22, 1926, to December 5, 1927 Map of the change to the United States in central North America on November 22, 1926
July 18, 1927 The U.S. expropriated from Panama another 33 hectares of land on the islands of Taboga and Taboguilla and annexed them to the Panama Canal Zone.[309] too small to map
October 26, 1927 Two bancos along the Colorado River were ceded from Mexico to Arizona.[333][334] too small to map
December 5, 1927 The "Country Club Dispute" between New Mexico and Texas was resolved in Texas's favor.[335] Map of the United States in central North America from December 5, 1927, to March 16, 1936 Map of the change to the United States in Central North America on December 5, 1927
April 4, 1928 The Island of Palmas Case was decided in the favor of the Netherlands, ceding Palmas to the Dutch East Indies.[275] Map of the United States in the Pacific Ocean from April 4, 1928, to December 13, 1932 Map of the change to the United States in the Pacific Ocean on April 4, 1928
September 24, 1928 The U.S. expropriated from Panama three hectares of land at El Cerro de Doscientos Pies ("200-Foot Hill") near Las Minas Bay and annexed it to the Panama Canal Zone.[309][316] too small to map
July 22, 1930 The U.S. expropriated from Panama 25 hectares on Jicarita Island and 60 hectares at Punta Morro de Puercos and annexed them to the Panama Canal Zone.[309] too small to map
April 15, 1931 The U.S. expropriated from Panama additional areas around the soon-to-be-built Madden Dam and annexed them to the Panama Canal Zone.[309][316] too small to map
May 3, 1932 The U.S. adjusted the border at Punta Paitilla in the Canal Zone, returning a small amount of land to Panama. This was the site for a planned new U.S. embassy, which had to be built on foreign soil.[336] too small to map
May 17, 1932 Porto Rico was renamed Puerto Rico.[337] Map of the United States in the Caribbean Sea from May 17, 1932, to April 11, 1955 Map of the change to the United States in the Caribbean Sea on May 17, 1932
December 13, 1932 The Mangsee Islands and seven of the Turtle Islands were ceded by the United Kingdom from North Borneo to the Philippines. The islands were supposed to be included in the 1900 transfer of islands from Spain to the United States. Per the terms of the treaty, the United Kingdom continued to administer the islands until requested, and after independence, the Philippine government made such a request and took control.[338] Map of the United States in the Pacific Ocean from December 13, 1932, to May 13, 1936 Map of the change to the United States in the Pacific Ocean on December 13, 1932
November 13, 1933 A treaty created the Rio Grande Rectification Project, which, from 1935 to 1938, straightened and stabilized the path of the Rio Grande through the El PasoJuárez Valley. By the end of the project, 174 parcels had been transferred between Mexico and Texas, each side receiving an equal area of land.[339][340] too small to map
December 29, 1934 Kingman Reef was placed under the jurisdiction of the U.S. Department of Defense.[325] no change to map
March 16, 1936 The de jure overlap between Michigan and Wisconsin was resolved by the Supreme Court of the United States.[341] Map of the United States in central North America from March 16, 1936, to the present day Map of the change to the United States in central North America on March 16, 1936
May 13, 1936 Baker Island, Howland Island, and Jarvis Island were formally annexed and placed under the jurisdiction of the U.S. Department of the Interior,[342] ending the United Kingdom's claim to Jarvis Island.[222] Map of the United States in the Pacific Ocean from May 13, 1936, to August 6, 1936 Map of the change to the United States in the Pacific Ocean on May 13, 1936
June 22, 1936 The United States Virgin Islands were organized into a civil territory.[343] no change to map
August 6, 1936 Canton Island, Enderbury Island, and McKean Island were claimed by the United Kingdom.[222] Map of the United States in the Pacific Ocean from August 6, 1936, to April 6, 1939 Map of the change to the United States in the Pacific Ocean on August 6, 1936
April 6, 1939 The condominium of the Canton and Enderbury Islands was established with the United Kingdom.[344] Map of the United States in the Pacific Ocean from April 6, 1939, to August 16, 1939 Map of the change to the United States in the Pacific Ocean on April 6, 1939
July 27, 1939 Panama gained a sovereign corridor that was carved out of the Panama Canal Zone connecting Colón with the rest of Panama, along with a three-dimensional "tube" of sovereignty for a future crossing over a U.S. highway. A corridor consisting of the road from the Canal Zone boundary to Madden Dam was annexed to the Canal Zone. too small to map
August 16, 1939 This is the earliest date so far discovered for when the United States began claiming Fakaofo, Funafuti, Hull Island, Niulakita, Nukufetau, and Nukulaelae.[345] Map of the United States in the Pacific Ocean from August 16, 1939, to August 16, 1939 Map of the change to the United States in the Pacific Ocean on August 16, 1939
December 10, 1941 Governor George McMillin surrendered Guam to the Japanese military.[346] Map of the United States in the Pacific Ocean from December 10, 1941, to December 23, 1941 Map of the change to the United States in the Pacific Ocean on December 10, 1941
December 23, 1941 The garrison on Wake Island surrendered to the Japanese military.[347] Map of the United States in the Pacific Ocean from December 23, 1941, to March 26, 1942 Map of the change to the United States in the Pacific Ocean on December 23, 1941
March 26, 1942 The government of the Commonwealth of the Philippines evacuated from the territory in the face of Japanese advance. A government-in-exile would be established in Washington, D.C. on May 17, 1942. The United States Army Forces in the Far East would surrender on April 9, 1942, following the Battle of Bataan, and the final military holdouts would surrender on May 6, 1942, following the Battle of Corregidor.[348] Map of the United States in the Pacific Ocean from March 26, 1942, to October 14, 1943 Map of the change to the United States in the Pacific Ocean on March 26, 1942
October 14, 1943 The Second Philippine Republic was established as a puppet state of Japan.[348] Map of the United States in the Pacific Ocean from October 14, 1943, to August 10, 1944 Map of the change to the United States in the Pacific Ocean on October 14, 1943
August 10, 1944 Guam was captured from Japan.[346] Map of the United States in the Pacific Ocean from August 10, 1944, to August 17, 1945 Map of the change to the United States in the Pacific Ocean on August 10, 1944
August 17, 1945 The Second Philippine Republic, in exile in Tokyo since April 3, 1945, was dissolved. The process of re-establishing the Commonwealth government on Philippine soil had started on October 23, 1944.[348] Map of the United States in the Pacific Ocean from August 17, 1945, to July 4, 1946 Map of the change to the United States in the Pacific Ocean on August 17, 1945
September 4, 1945 The Japanese garrison on Wake Island surrendered to the United States.[347] Map of the United States in the Pacific Ocean from September 4, 1945, to July 4, 1946 Map of the change to the United States in the Pacific Ocean on September 4, 1945
July 4, 1946 The Commonwealth of the Philippines became independent as the Republic of the Philippines.[349] Map of the United States in the Pacific Ocean from July 4, 1946, to July 18, 1947 Map of the change to the United States in the Pacific Ocean on July 4, 1946
July 18, 1947 The United Nations entrusted the Trust Territory of the Pacific Islands to the United States.[350] Map of the United States in the Pacific Ocean from July 18, 1947, to January 1, 1949 Map of the change to the United States in the Pacific Ocean on July 18, 1947
January 1, 1949 The Tokelau Islands were incorporated into New Zealand, which inherited the claims on Atafu, Fakaofo, and Nukunono.[351] Map of the United States in the Pacific Ocean from January 1, 1949, to August 21, 1959 Map of the change to the United States in the Pacific Ocean on January 1, 1949
August 1, 1950 Guam was organized into a civil territory.[352][353] no change to map
August 3, 1950 Kansas and Missouri exchanged small portions of land along the Missouri River, due to shifts in the river following a flood in 1944.[354] Map of the United States in central North America from March 16, 1936, to the present day Map of the change to the United States in central North America on August 3, 1950
April 11, 1955 Panama's corridor connecting Colón with the rest of Panama was realigned within the Panama Canal Zone. Several three-dimensional "tubes" of sovereignty were also created, allowing Panamanian bridges to pass over rivers and a highway at several locations within the Canal Zone.[355][356] Map of the United States in the Caribbean Sea from April 11, 1955, to August 23, 1955 Map of the change to the United States in the Caribbean Sea on April 11, 1955
August 23, 1955 Several border locations of the Panama Canal Zone were redefined. Punta Paitilla, the land held on Taboga Island, and the remaining American holdings in Colón and Panama City were ceded to Panama.[287][357] Map of the United States in the Caribbean Sea from August 23, 1955, to September 1, 1972 Map of the change to the United States in the Caribbean Sea on August 23, 1955
January 3, 1959 Alaska Territory was admitted as the 49th state, Alaska.[358] Map of the United States in northwest North America from January 3, 1959, to the present Map of the change to the United States in northwest North America on January 3, 1959
August 21, 1959 Most of Hawaii Territory was admitted as the 50th state, Hawaii. Palmyra Atoll was excluded from statehood and remained a territory.[211][359] Map of the United States in the Pacific Ocean from August 21, 1959, to August 4, 1965 Map of the change to the United States in the Pacific Ocean on August 21, 1959
August 25, 1961 About 20 acres of land was transferred from Minnesota to North Dakota near Fargo, North Dakota;[159][360] since the area was very small and poorly documented, it is not mapped. too small to map
January 14, 1964 The Chamizal, a tract of land between El Paso, Texas, and Ciudad Juárez, Chihuahua, was divided between the United States and Mexico.[361] Map of the United States in central North America from March 16, 1936, to the present day Map of the change to the United States in central North America on January 14, 1964
August 4, 1965 The Cook Islands became self-governing from New Zealand. It claimed the atolls of Pukapuka, Manihiki, Penrhyn, and Rakahanga.[362] Map of the United States in the Pacific Ocean from August 4, 1965, to October 1, 1978 Map of the change to the United States in the Pacific Ocean on August 4, 1965
July 14, 1970 The lease of the Corn Islands from Nicaragua was terminated.[305] no change to map
September 1, 1972 The Swan Islands were ceded to Honduras.[305][363] Map of the United States in the Caribbean Sea from September 1, 1972, to October 1, 1979 Map of the change to the United States in the Caribbean Sea on September 1, 1972
May 26, 1977 Several parcels were ceded between Texas and Mexico along the Rio Grande,[364] including the Horcón Tract, on which the town of Río Rico was located.[365] Map of the United States in central North America from March 16, 1936, to the present day Map of the change to the United States in central North America on May 26, 1977
October 1, 1978 Tuvalu became independent from the United Kingdom. It claimed the atolls of Funafuti, Nukufetau, Nukulaelae, and Niulakita.[366] Map of the United States in the Pacific Ocean from October 1, 1978, to July 12, 1979 Map of the change to the United States in the Pacific Ocean on October 1, 1978
July 12, 1979 The Republic of Kiribati became independent from the United Kingdom. It claimed Birnie Island, Canton Island, Caroline Island, Christmas Island, Enderbury Island, Flint Island, Gardner Island, Hull Island, Malden Island, McKean Island, Phoenix Island, Starbuck Island, Sydney Island, and Vostok Island. This dissolved the condominium of the Canton and Enderbury Islands.[367] Map of the United States in the Pacific Ocean from July 12, 1979, to September 3, 1983 Map of the change to the United States in the Pacific Ocean on July 12, 1979
October 1, 1979 The Panama Canal Zone was ceded to Panama. The U.S. and Panama continued to share operational control of the canal until December 31, 1999, when it would be fully turned over to Panama.[368] The U.S. retained control over several hundred specified areas to be turned over in piecemeal fashion over the years. Map of the United States in the Caribbean Sea from October 1, 1979, to September 17, 1981 Map of the change to the United States in the Caribbean Sea on October 1, 1979
November 24, 1980 The maritime border between the United States and Venezuela was defined.[369] no change to map
September 17, 1981 Roncador Bank and Serrana Bank were ceded to Colombia, and the claim on Quita Sueño Bank was abandoned by the United States, as it was no longer above the seas at high tide and thus the government considered it unclaimable.[305][370] Map of the United States in the Caribbean Sea from September 17, 1981, to the present Map of the change to the United States in the Caribbean Sea on September 17, 1981
September 3, 1983 Atafu, Fakaofo, and Nukunono were ceded to New Zealand.[305][371] Map of the United States in the Pacific Ocean from September 3, 1983, to September 8, 1983 Map of the change to the United States in the Pacific Ocean on September 3, 1983
September 8, 1983 Pukapuka, Manihiki, Penrhyn, and Rakahanga were ceded to the Cook Islands.[305][372] Map of the United States in the Pacific Ocean from September 8, 1983, to September 23, 1983 Map of the change to the United States in the Pacific Ocean on September 8, 1983
September 23, 1983

Birnie Island, Canton Island, Caroline Island, Christmas Island, Enderbury Island, Flint Island, Gardner Island, Hull Island, Malden Island, McKean Island, Phoenix Island, Starbuck Island, Sydney Island, and Vostok Island were ceded to Kiribati.[305][373]

Funafuti, Nukufetau, Nukulaelae, and Niulakita were ceded to Tuvalu.[305][374]

Map of the United States in the Pacific Ocean from September 23, 1983, to October 21, 1986 Map of the change to the United States in the Pacific Ocean on September 23, 1983
October 12, 1984 The International Court of Justice made its judgment on where the maritime border should be in the Gulf of Maine between the United States and Canada.[375] No land changed hands. no change to map
October 21, 1986 The Marshall Islands District of the Trust Territory of the Pacific Islands became independent as the Republic of the Marshall Islands.[376] The Marshall Islanders had claimed Wake Island as part of their territory since at least 1973, and continued that after independence.[377] Map of the United States in the Pacific Ocean from October 21, 1986, to November 3, 1986 Map of the change to the United States in the Pacific Ocean on October 21, 1986
November 3, 1986 Most of the Trust Territory of the Pacific Islands was dissolved by the United Nations. The districts of Chuuk, Pohnpei, and Yap became independent as the Federated States of Micronesia. The Mariana Islands District, having already been taking moves towards integration with the U.S., became a territory of the United States, the Commonwealth of the Northern Mariana Islands.[376] Map of the United States in the Pacific Ocean from November 3, 1986, to October 1, 1994 Map of the change to the United States in the Pacific Ocean on November 3, 1986
June 1, 1990 The maritime border between the United States and the Soviet Union was provisionally defined. The two countries agreed on this date to abide by the terms of the treaty pending its ratification and entry into force,[378] but while it was ratified by the U.S. Senate on September 16, 1991,[379] it is unknown if either the Soviet Union or its successor state, Russia, ratified it. no change to map
October 1, 1994 The remaining district of the Trust Territory of the Pacific Islands, the Palau District, became independent as the Republic of Palau, dissolving the TTPI.[380] Map of the United States in the Pacific Ocean from October 1, 1994, to the present Map of the change to the United States in the Pacific Ocean on October 1, 1994
June 1, 1995 The maritime border between the United States and territories of the United Kingdom in the Caribbean Sea was defined.[381][382] no change to map
January 16, 1997 Navassa Island was transferred to the U.S. Department of the Interior.[383][384] no change to map
November 13, 1997 The maritime border between the United States and Mexico was defined.[385] no change to map
December 31, 1999 All former Panama Canal Zone parcels not turned over since 1979, as well as all joint canal operations areas, were transferred to Panama. too small to map
January 17, 2001 The maritime border between the United States and Mexico on the continental shelf in the western Gulf of Mexico beyond 200 nautical miles was defined.[386] no change to map
November 24, 2009 Six islands along the Rio Grande were ceded from Texas to Mexico, and 3 islands and 2 bancos were ceded from Mexico to Texas. The transfer, which had been pending for 20 years, was the first application of Article III of the 1970 Boundary Treaty.[334][387] too small to map
September 23, 2014 The maritime border between the United States and Niue was defined.[388] The treaty was signed on May 13, 1997, but it wasn't ratified by the United States until at least 2002, and the United Nations shows it as entering into force on this date.[389] no change to map

Bancos along the Rio Grande[edit]

The Banco Convention of 1905 between the United States and Mexico[390] allowed, in the event of sudden changes in the course of the Rio Grande (as by flooding), for the border to be altered to follow the new course. The sudden changes often created bancos, land left behind when curves in the river are cut off by rapid erosion of the channel or are intentionally cut to re-align it. When these bancos are created, the International Boundary and Water Commission investigates if land previously belonging to the U.S. or Mexico is to be considered on the other side of the border.[391] In all cases of these adjustments thus far, the transferred land is minuscule (ranging in size from 1 acre to 646 acres) and uninhabited. Due to the small size and difficulty in determining exactly which land was ceded, these are not listed here; please see the articles linked above for more detail. This information is also available as a kmz file mapped at this location.

See also[edit]

References[edit]

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Further reading[edit]