In 1845, the United States of America annexed the Republic of Texas and admitted it to the Union as the 28th state. The U.S. thus inherited Texas' border dispute with Mexico; this quickly led to the Mexican-American War, during which the U.S. captured additional territory (known as the Mexican Cession of 1848), extending the nation's borders all the way to the Pacific Ocean. Texas claimed the eastern part of this new territory, comprising parts of present-day Colorado, Kansas, New Mexico, Texas, Wyoming, Utah, and Oklahoma. The resulting dispute among Texas, the federal government, and New Mexico Territory was resolved in the Compromise of 1850, when much of these lands became parts of other territories of the United States in exchange for the U.S. federal government assuming the Texas Republic's $10 million in debt.
Anglo-American immigrants, primarily from the Southern United States, began emigrating to Mexican Texas in the early 1820s at the request of the Mexican government, which sought to populate the sparsely inhabited lands of its northern frontier. Anglo-Americans soon became a majority in Texas and eventually became disillusioned with Mexican rule. Coahuila y Tejas, a Mexican state of which Texas was a constituent part after 1824, endorsed a plan for the gradual emancipation of the state's slaves in 1827, which angered many slaveholding settlers who had moved to Texas from the South. For this and other reasons, Texas declared independence from Mexico, resulting in war with Mexico. In 1836, the fighting ended and Sam Houston became the first president of the Republic of Texas, elected on a platform that favored annexation to the United States.
Initial Texan proposal 
In August 1837, Memucan Hunt, Jr., the Texan minister to the United States, submitted an annexation proposal to the Van Buren administration. Believing that annexation would lead to war with Mexico, the administration declined Texas’ proposal. After the election of Mirabeau B. Lamar, an opponent of annexation, as president of Texas in 1838 and the United States’ apprehension regarding annexation, Texas withdrew its offer.
Failed treaty 
In 1843, President John Tyler came out in support of annexation, entering negotiations with the Republic of Texas for an annexation treaty, which he submitted to the Senate. On June 8, 1844, the treaty was defeated 16 to 35, well below the two-thirds majority necessary for ratification. Of the 29 Whig senators, 28 voted against the treaty with only one Whig, a southerner, supporting it. The Democratic senators were more divided on the issue with six northern Democrats and one southern Democrat opposing the treaty and five northern Democrats and ten southern Democrats supporting it.
Annexation by joint resolution 
James K. Polk, a Democrat and a strong supporter of territorial expansion, was elected president in November 1844 with a mandate to acquire both the Republic of Texas and Oregon Country. After the election, the Tyler administration realized that public opinion was in favor of annexation, consulted with President-elect Polk, and set out to accomplish annexation by means of a joint resolution. The resolution declared that Texas would be admitted as a state as long as it approved annexation by January 1, 1846, that it could split itself up into five additional states, and that possession of the Republic’s public land would shift to the state of Texas upon its admission. On February 28, 1845, four days before Polk took office, Congress passed the joint resolution. Not long afterward, Andrew Jackson Donelson, the American chargé d'affaires in Texas and the nephew of former president Andrew Jackson, presented the American resolution to President Anson Jones of Texas. On July 4, 1845, the Texan Congress endorsed the American annexation offer with only one dissenting vote and began writing a state constitution. The citizens of Texas approved the new constitution and the annexation ordinance on October 13, 1845, and Polk signed the documents formally integrating Texas into the United States on December 29, 1845.
Options for the formation of new states 
The joint resolution and ordinance of annexation contain language permitting the formation of up to four additional states out of the former territories of the Republic of Texas:
New States of convenient size not exceeding four in number, in addition to said State of Texas and having sufficient population, may, hereafter by the consent of said State, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the Federal Constitution. 
The joint resolution required that if any new states were formed out of Texas’ lands, those north of the Missouri Compromise line would become free states and those south of the line could choose whether or not to permit slavery. Article Four of the Constitution allows the creation of new states out of an existing one with the consent of both the legislature of that state and of Congress.
Border disputes 
The joint resolution and ordinance of annexation have no language specifying the boundaries of Texas, but only refer in general terms to "the territory properly included within, and rightfully belonging to the Republic of Texas", and state that the new State of Texas is to be formed "subject to the adjustment by this [U.S.] government of all questions of boundary that may arise with other governments."
According to George Lockhart Rives, "That treaty had been expressly so framed as to leave the boundaries of Texas undefined, and the joint resolution of the following winter was drawn in the same manner. It was hoped that this might open the way to a negotiation, in the course of which the whole subject of the boundaries of Mexico, from the Gulf to the Pacific, might be reconsidered, but these hopes came to nothing."
There was an ongoing border dispute between the Republic of Texas and Mexico prior to annexation. Texas claimed the Rio Grande as its border, while Mexico maintained that it was the Nueces River and did not recognize Texan independence. President James K. Polk ordered General Zachary Taylor to garrison the southern border of Texas, as defined by the former Republic. Taylor moved into Texas, ignoring Mexican demands to withdraw, and marched as far south as the Rio Grande, where he began to build a fort near the river's mouth on the Gulf of Mexico. The Mexican government regarded this action as a violation of its sovereignty.
Though the Republic of Texas claimed the Rio Grande as its southern and western border, it never controlled what is now New Mexico. The failed Texas Santa Fe Expedition of 1841 was its only attempt to take that territory. El Paso was only taken under Texas governance by Robert Neighbors in 1850, over four years after annexation; he was not welcomed in New Mexico. Texas continued to claim New Mexico as far as the Rio Grande, supported by the rest of the South, and opposed by the North and by New Mexico itself, until agreeing to today's boundary in the Compromise of 1850.
Legality controversy 
The formal controversy about the legality of the annexation of Texas stems from the fact that Congress approved the annexation of Texas as a territory with a simple majority vote approval instead of annexing the land by Treaty, as was done with Native American lands. After the United States and The Republic of Texas were unable to reach a Treaty agreement, Congress passed a Joint Resolution for Annexing Texas to the United States. The Republic of Texas's Annexation Convention then submitted the a Joint Resolution for Annexing Texas to the United States to popular vote in October 1845 and the public approved the measure. This Ordinance of Annexation was submitted and approved by the US House and Senate and signed by the President on December 29, 1845. While this was an awkward, if not unusual, treaty process it was fully accepted by all parties involved, and more importantly all parties performed on those agreements making them legally binding (see Contract law). In addition, the United States Supreme Court decided in the case of DeLima v. Bidwell, 182 U.S. 1 (1901), that annexation by a joint resolution of Congress was legal.
See also 
- Ray Allen Billington,The Far Western Frontier, 1830-1860 (New York: Harper & Row, 1956), p. 116.
- Op. cit. Billington 1956, pp. 116-117.
- Alvin Laroy Duckett (2010). John Forsyth: Political Tactician. Athens: University of Georgia Press. p. 201. ISBN 0-8203-3534-7.
- Richard Bruce Winders, Crisis in the Southwest: The United States, Mexico, and the Struggle over Texas (Lanham: Rowman & Littlefield, 2002), p. 41.
- T.R. Fehrenbach, Lone Star: A History of Texas and Texans (Cambridge: Da Capo Press, 2000), p. 263.
- Thomas M. Hietala, Manifest Design: American Exceptionalism and Empire (Ithaca: Cornell University Press, 1985), p. 40.
- Fehrenbach, Lone Star, p. 264
- Op. cit. Fehrenbach Lone Star, p. 265
- Op. cit. Fehrenbach Lone Star, p. 266; Hietala, Manifest Design, p. 43.
- Fehrenbach, Lone Star, p. 266.
- Op. cit. Fehrenbach Lone Star, p. 267
- "Joint Resolution of Congress, Section 2, March 1, 1845".
- "Section 3 of Ordinance of the Convention of Texas, signed 1845 July 4".
- Joint Resolution of Congress, Mar. 1, 1845
- George Lockhart Rives (1913). The United States and Mexico, 1821-48 2. C. Scribner's Sons. p. 47.
- Compromise of 1850, The Handbook of Texas Online.
- Joint Resolution for Annexing Texas to the United States
- Peters, Richard, ed., The Public Statutes at Large of the United States of America, v.5 (Boston: Chas C. Little & Jas. Brown, 1850), 797–798.
- Ordinance of Annexation
- Journals of the Constitution Convention of Texas (Austin: Miner & Cruger, Printers to the Constitution, 1845) 367–370.
- De Lima v. Bidwell 182 U.S. 1 (1901). FindLaw.
Primary sources 
- Avalon Project at Yale Law School. Texas from Independence to Annexation.
- Joint Resolution for Annexing Texas to the United States, Approved March 1, 1845.
- Jones, Anson. Letters, Relating to the History of Annexation. Galveston: Civilian Office, 1848.
- Ordinance of Annexation, Approved by the Texas Convention on July 4, 1845.
- Dewey, Orville. Discourse on Slavery and the Annexation of Texas. New York: C.S. Francis & Co., 1844.
Secondary sources 
- "Annexation." Handbook of Texas Online.
- Carefoot, Jean. “Narrative History of Texas Annexation.” Texas State Library and Archives Commission.
- Silbey, Joel H. Storm over Texas: The Annexation Controversy and the Road to Civil War. New York: Oxford University Press, 2005.
- Smith, Justin H. The Annexation of Texas. Unknown publisher, 1911.