Date: From 1817 to 1820
When the territory acquired through the Louisiana Purchase was added to the United States in 1803, no provisions were made for the existence of slavery in new states created out of the new acquisition. In 1812, Louisiana was the first state formed out of the Louisiana Purchase but the predominance of slavery there made the status of the institution within its borders uncontroversial. However, Missouri's application for statehood in 1817 provoked a nationwide debate over the status of slavery in the American West. Located along the sectional border between the North and South, Missouri was divided between slave and free regions, so its status was not as easily determined as Louisiana and politicians were aware that whatever decision they made concerning the state would set precedent for the admission of future states west of the Mississippi River. The crisis began in earnest when New York Representative James Tallmadge proposed an amendment to the statehood bill preventing any further slaves from entering the new state and freeing the children of slaves already living in Missouri when they turned twenty-five. The resulting division in Congress was stark and conformed largely to regional interests, rather than party loyalty. The measure passed the House of Representatives but failed in the Senate. The following year, Illinois Senator Jesse B. Thomas proposed what became the Missouri Compromise. Congress would admit Missouri as a slave state but maintained balance in the Senate by admitting Maine as a free state. In addition, the southern border of Missouri (36 30 north latitude) would be extended West as a dividing line between future slave states on the southern side and free states to the north. The compromise initially failed but eventually passed Congress as a set of separate measures. The Missouri Compromise effectively maintained sectional harmony within the United States until debates over the status of territories acquired from the Mexican War rekindled the conflict and eventually led to the Civil War.
Robert Pierce Forbes, The Missouri Compromise and its Aftermath: Slavery and the Meaning of America (Chapel Hill: University of North Carolina Press, 2007); William W. Freehling, The Road to Disunion: Secessionists at Bay, 1776-1854 (Oxford: Oxford University Press, 1990), 144-161; Glover Moore, The Missouri Controversy, 1819-1821 (Lexington: University of Kentucky Press, 1953).