Scott v. Sandford

When Missouri applied for admittance to the Union in 1819, the United States was equally divided between free and slave states, with 11 of each. Since slavery already existed in Missouri Territory, it sought admission as a slave state, which infuriated New England abolitionists and other opponents of slavery. Southerners were equally incensed when a ban on the importation of slaves was suggested as a precondition for Missouri's statehood.

Henry Clay of Kentucky offered a compromise: since Maine had also requested admission to the Union, he proposed that Missouri enter as a slave state and Maine be admitted as a free state. The existing parity between free and slave states would remain. Moreover, according to the proposal which became the Missouri Compromise, an imaginary line would be drawn at the 36 degrees, 30 minutes north parallel--the southern boundary of Missouri. No territory above that line could be admitted as a slave state, and no territory below it could enter as a free state.

The Missouri Compromise was repealed by the Kansas-Nebraska Act in 1854, which introduced popular sovereignty over the slavery issue.