On 17 February 1821, the Senate passed the Missouri Compromise, an attempt to deal with the dangerously divisive issue of extending slavery into the western territories.

From colonial days to the Civil War, slavery and western expansion both played fundamental but inherently incompatible roles in the American republic. As the nation expanded westward, the Congress adopted relatively liberal procedures by which western territories could organize and join the union as full-fledged states. Southern slaveholders, eager to replicate their plantation system in the West, wanted to keep the new territories open to slavery. Abolitionists, concentrated primarily in the industrial North, wanted the West to be exclusively a free labor region and hoped that slavery would gradually die out if confined to the South. Both factions realized their future congressional influence would depend on the number of new “slave” and “free” states admitted into the union.

Consequently, the West became the first political battleground over the slavery issue. In 1818, the Territory of Missouri applied to Congress for admission as a slave state. Early in 1819, a New York congressman introduced an amendment to the proposed Missouri constitution that would ban importation of new slaves and require gradual emancipation of existing slaves. Southern congressmen reacted with outrage, inspiring a nationwide debate on the future of slavery in the nation.

Over the next year, the congressional debate grew increasingly bitter, and southerners began to threaten secession and civil war. To avoid this disastrous possibility, key congressmen hammered together an agreement that became known as the Missouri Compromise. In exchange for admitting Missouri without restrictions on slavery, the Compromise called for bringing in Maine as a free state. The Compromise also dictated that slavery would be prohibited in all future western states carved out of the Louisiana Territory that were higher in latitude than the northern border of Arkansas Territory.

Although the Missouri Compromise temporarily eased the inherent tensions between western expansion and slavery, the divisive issue was far from resolved. Whether or not to allow slavery in the states of Texas, Kansas, and Nebraska caused the same difficulties several decades later, leading the nation toward civil war.