Slave and free states
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In the United States of America prior to the American Civil War, a slave state was a U.S. state in which slavery was legal, whereas a free state was one in which slavery was either prohibited from its entry into the Union or eliminated over time. Slavery was one of the causes of the American Civil War and was abolished by the 13th Amendment of the United States Constitution in 1865.
The Northeastern and Mid-Atlantic States (colonies), including Massachusetts, New York, Pennsylvania, and New Jersey, had legally permitted slavery in the 17th, 18th, and even part of the 19th centuries, but in the generation or two before the American Civil War, almost all slaves in such states had been emancipated through a series of statutes.
The first U.S. region by federal law that prohibited slavery was the Northwest Territory, which was ordained free under the Northwest Ordinance of 1787, passed just before the U.S. Constitution was ratified. The states created from this region—Ohio, Indiana, Michigan, Illinois, Wisconsin, and Minnesota —were generally settled by New Englanders and American Revolutionary War veterans granted land there. Because in this region slavery was prohibited from its inception and separated by the Ohio River from the South—which was pushing an expansion of legal slavery into the west—the concept developed of "free states" in contrast to "slave states." The rural parts of these states, at one time in direct East-West rivalry with the Northeastern commercial states, realigned with the Northeastern states, which were newly free of slavery, and together these regions created the amalgamation of states prohibiting slavery, known in the context of the Civil War as the free states.
Original state-based abolition efforts 
Prior to the American Revolution, all of the British North American colonies had slavery, but the Revolutionary War gave impetus to a general anti-slavery sentiment. The Northwest Territory, now known as part of the Midwest, was organized under the Northwest Ordinance with a prohibition on slavery in 1787. Massachusetts accepted that its 1780 Constitution effectively abolished slavery, and several other northern states adopted statutes requiring gradual emancipation. In 1804, New Jersey became the last original state to embark on the course of gradual emancipation.
Conflict over new territories 
During the War of 1812, the British accepted as free all slaves who came into their hands, with no conditions as to military service such as had been made in Dunmore's Proclamation in the Revolutionary War. By the end of the War of 1812, the momentum for antislavery reform, state by state, appeared to run out of steam, with half of the states having already abolished slavery (Northeast), prohibited from the start (Midwest) or committed to eliminating slavery, and half committed to continuing the institution indefinitely (South).
The potential for political conflict over slavery at a federal level made politicians concerned about the balance of power in the U.S. Senate, where each State was represented by two Senators. With an equal number of slave states and free states, the United States Senate was equally divided. As the population of the free states began to outstrip the population of the slave states, leading to control of the House of Representatives by free states, the Senate became the preoccupation of slave state politicians interested in maintaining a Congressional veto over federal policy in regard to slavery. As a result of this preoccupation, slave states and free states were often admitted into the Union in pairs to maintain the existing Senate balance between slave and free.
Missouri Compromise 
Controversy over whether Missouri should be admitted as a slave state, resulted in the Missouri Compromise of 1820, which specified that Louisiana Purchase territory north of latitude 36° 30', which described Missouri's southern boundary, would be organized as free states and territory south of that line would be reserved for organization as slave states. As part of that compromise, the admission of Maine as a free state was secured to balance Missouri's admission as a slave state.
Status of Texas and the Mexican Cession states 
The admission of Texas and the acquisition of vast new western territories after the Mexican-American War further excited controversy. Although the settled portion of Texas was an area rich in cotton plantations and dependent on slavery, the territory acquired in the Mountain West did not seem hospitable to cotton or slavery. In 1850, California was admitted as a free state, without an additional slave state as balance. This would have created a free state majority in the Senate, except that California agreeably sent one pro-slavery and one anti-slavery senator to Washington, D.C. Thus, the admission of California increased the anxiety of pro-slavery politicians but did not change the balance in the Senate.
Last battles 
The difficulty of identifying territory that could be organized into additional slave states stalled the process of opening the western territories to settlement, while slave state politicians sought a solution. Efforts were made to acquire Cuba and to annex Nicaragua, both to be slave states. In 1854, the Missouri Compromise of 1820 was repealed, and an effort was initiated to organize Kansas as a slave state. Kansas was paired with Minnesota for admission, but the admission of Kansas as a slave state was blocked because of questions over the legitimacy of its slave state constitution. When the admission of Minnesota proceeded unimpeded in 1858, the balance in the Senate was lost; a loss that was compounded by the subsequent admission of Oregon in 1859.
Slave and free state pairs 
Before 1812, the concern about balancing slave-states and free states was not profound. This is how the states lined up in 1812:
|Slave States||Year||Free States||Year|
(Slave until 1804)
|North Carolina||1789||New York
(Slave until 1799)
After 1812, and until the Civil War, maintaining the balance of free and slave states within the federal legislature was considered of paramount importance if the Union were to be preserved, and states were typically admitted in pairs:
|Slave States||Year||Free States||Year|
(One pro-slavery Senator)
End of slave states 
Maryland, Missouri, Tennessee, the new state of West Virginia, and the District of Columbia prohibited slavery before the Civil War ended. However, in Delaware, Kentucky, and most of the Confederate States, slavery continued to be legal until the 13th Amendment to the United States Constitution abolished slavery throughout the United States in December 1865, ending the distinction. Ratification of the 13th Amendment was a condition of the return of local rule to those states that had declared their secession.
See also 
- Border states (Civil War)
- Confederate States of America
- Slavery in the United States
- Slavery in the colonial United States
- "Archives of Maryland Historical List: Constitutional Convention, 1864". November 1, 1864. Retrieved 2012-11-18.
- "Missouri abolishes slavery". January 11, 1865. Retrieved 2012-11-18.
- "TENNESSEE STATE CONVENTION: Slavery Declared Forever Abolished". NY Times. January 14, 1865. Retrieved 2012-11-18.
- "On this day: 1865-FEB-03". Retrieved 2012-11-18.
- "Slavery in Delaware". Retrieved 2012-11-18.
- Lowell Hayes Harrison and James C. Klotter (1997). A new history of Kentucky. p. 180. In 1866, Kentucky refused to ratify the 13th Amendment. It did ratify it in 1976.
- Don E. Fehrenbacher and Ward M. Mcafee; The Slaveholding Republic: An Account of the United States Government's Relations to Slavery (2002)