1820–21, measures passed by the U.S. Congress to end the first of a series of crises concerning the extension of slavery.

By 1818, Missouri Territory had gained sufficient population to warrant its admission into the Union as a state. Its settlers came largely from the South, and it was expected that Missouri would be a slave state. To a statehood bill brought before the House of Representatives, James Tallmadge of New York proposed an amendment that would forbid importation of slaves and would bring about the ultimate emancipation of all slaves born in Missouri. This amendment passed the House (Feb., 1819), but not the Senate. The bitterness of the debates sharply emphasized the sectional division of the United States.

In Jan., 1820, a bill to admit Maine as a state passed the House. The admission of Alabama as a slave state in 1819 had brought the slave states and free states to equal representation in the Senate, and it was seen that by pairing Maine (certain to be a free state) and Missouri, this equality would be maintained. The two bills were joined as one in the Senate, with the clause forbidding slavery in Missouri replaced by a measure prohibiting slavery in the remainder of the Louisiana Purchase north of 36°30′N lat. (the southern boundary of Missouri). The House rejected this compromise bill, but after a conference committee of members of both houses was appointed, the bills were treated separately, and in Mar., 1820, Maine was made a state and Missouri was authorized to adopt a constitution having no restrictions on slavery.

A provision in the Missouri constitution barring the immigration of free blacks to the state was objectionable to many Northern Congressmen, and necessitated another congressional compromise. Not until the Missouri legislature pledged that nothing in its constitution would be interpreted to abridge the rights of citizens of the United States was the charter approved and Missouri admitted to the Union (Aug., 1821). Henry Clay, as speaker of the House, did much to secure passage of the compromise—so much, in fact, that he is generally regarded as its author, even though Senator Jesse B. Thomas of Illinois was far more responsible for the first bill. The 36°30′ proviso held until 1854, when the Kansas-Nebraska Act repealed the Missouri Compromise.

See studies by G. Moore (1953, repr. 1967) and R. H. Brown (1964).

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