1 Original state-based abolition efforts

All of the British North American colonies, prior to the Revolution, had slavery, but the Revolutionary War gave impetus to a general antislavery sentiment. The Northwest Territories were organized, with a prohibition on slavery. Massachusetts accepted that its 1780 Constitution effectively abolished slavery, and several other northern States adopted statutes requiring gradual emancipation. New Jersey was the last State to embark on the course of gradual emancipation, in 1804, a process, which had still not quite eliminated slavery entirely, even as late as 1860.

2 Conflict over new territories

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 or committed to eliminating slavery, and half committed to continuing the institution indefinitely. The potential for political conflict over slavery at a federal level led politicians to be 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 Senate was equally divided. As the population of the free States began to outstrip the population of the slave States, the Senate became the preoccupation of Slave state politicians, interested in maintaining a 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, so as to maintain the existing balance between slave and free.

2.1 The admission of Missouri and the 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 was secured to balance Missouri's admission as a slave State.

2.2 Status of Texas and the Mexican Cession states

The admission of Texas and the acquisition of vast new western territories in the Mexican 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 for the fact that California agreeably sent one pro-slavery and one anti-slavery Senator to Washington. Thus, the admission of California increased the anxiety of pro-slavery politicians, but did not change the balance in the Senate.

2.3 The last battles

The difficulty of identifying any territory, which 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 even to annex Nicaragua. 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, over the legitimacy of its Slave state constitution. When the admission of Minnesota proceeded unimpeded, the balance in the Senate was lost, a loss compounded by the subsequent admission of Oregon.

3 The end of slave states

Maryland and Missouri abolished slavery during the Civil War.

The Thirteenth Amendment to the United States Constitution, ratified December 6, 1865, abolished slavery throughout the United States, ending the distinction, Slave state.

4 Slave and free state pairs

Slave State(s)	Year	Free State(s)	Year
Kentucky	1792	Vermont	1791
Tennessee	1796	Ohio	1803
Louisiana	1812
Mississippi	1817	Indiana	1816
Alabama	1819	Illinois	1818
Missouri	1821	Maine	1820
Arkansas	1836	Michigan	1837
Florida	1845	Iowa	1846
Texas	1845	Wisconsin	1848
		California	1850
Kansas (delayed)		Minnesota	1858
		Oregon	1859
		Kansas	1861

U.S. history of slavery

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