What did the Missouri Compromise do?

The Missouri Compromise of 1820 admitted Missouri to the Union as a slave state and Maine as a free state to preserve the Senate balance, banned slavery from the Louisiana Purchase territory north of latitude 36 degrees 30 minutes (the southern boundary of Missouri), and set the precedent that Congress could regulate slavery in the territories through legislative compromise. The threefold action of the compromise unfolded in three connected statutes.

First, it admitted Maine to the Union as the 23rd state on March 15, 1820. Maine had been the northern district of Massachusetts since the colonial period, and Massachusetts agreed in June 1819 to allow Maine to seek separate statehood. Maine's admission as a free state offset the impending admission of Missouri as a slave state and preserved the Senate balance at 12 free and 12 slave states.

Second, it authorized Missouri to write a constitution and form a state government, with admission to follow. Missouri became the 24th state on August 10, 1821 after the Second Missouri Compromise resolved questions about its provision restricting free Black migration. Missouri's admission as a slave state provided the territorial expansion of slavery that Southern senators required and that they had been threatening to block all northern legislation to obtain.

Third, the compromise included a geographic provision banning slavery in the unorganized portions of the Louisiana Purchase territory north of latitude 36 degrees 30 minutes, except within Missouri itself. The 36 degrees 30 minutes line ran along Missouri's southern border, dividing the trans-Mississippi West along that latitude. The provision implied that future states formed north of the line would enter as free states and those south of the line could enter as slave states. Iowa would be admitted as free in 1846, Wisconsin as free in 1848, Minnesota as free in 1858, Kansas as free in 1861, all north of the line. Arkansas was admitted as slave in 1836 and Texas (annexed not from the Louisiana Purchase) as slave in 1845, both south of the line in spirit.

Speaker of the House Henry Clay of Kentucky earned the nickname "the Great Compromiser" for his role in negotiating the bargain. President James Monroe signed the bill on March 6, 1820. The compromise held for 34 years.

The Compromise of 1850 partly modified the framework. The Kansas-Nebraska Act of May 30, 1854, championed by Stephen Douglas, repealed the line by allowing settlers in those territories to decide on slavery through popular sovereignty regardless of latitude. The Supreme Court in Dred Scott v. Sandford on March 6, 1857 ruled the original 36 degrees 30 minutes ban unconstitutional, reasoning that Congress could not deprive citizens of property (enslaved persons) without due process.