What was the Missouri Compromise?

The Missouri Compromise was a series of laws passed in 1820 and 1821 that admitted Missouri to the Union as a slave state and Maine as a free state, preserving the Senate balance between slave and free states, while also banning slavery from any future state to be carved from the Louisiana Purchase territory north of latitude 36 degrees 30 minutes north (the southern boundary of Missouri).

The crisis began in February 1819 when Missouri applied for statehood with a constitution allowing slavery. James Tallmadge of New York proposed an amendment in the House of Representatives that would have prohibited additional slaves from entering Missouri and freed those born there at age 25. The Tallmadge Amendment passed the House but died in the Senate, and the controversy over Missouri statehood paralyzed Congress for more than a year. By 1819 the Union contained 11 free states and 11 slave states, giving each side equal representation in the Senate. The admission of Missouri threatened to upset that balance.

Massachusetts had agreed in 1819 to allow its northern district of Maine to seek separate statehood. Speaker of the House Henry Clay of Kentucky engineered the compromise during the winter of 1819 to 1820. The basic terms paired Missouri's admission as a slave state with Maine's admission as a free state, preserving Senate parity at 12 each. The compromise also drew a geographic line at 36 degrees 30 minutes north latitude across the Louisiana Purchase territory: slavery would be allowed south of the line and prohibited north of it, except within Missouri itself. The Senate passed the compromise in February 1820 and the House on March 2, 1820. President James Monroe signed it on March 6, 1820.

Missouri's actual admission was delayed by a second crisis when Missouri's proposed state constitution attempted to bar free Black migration, raising questions about the privileges and immunities of free Black citizens of other states. Clay brokered a Second Missouri Compromise on March 2, 1821 requiring Missouri to promise that no law would be passed in violation of the U.S. Constitution. Maine became the 23rd state on March 15, 1820 and Missouri the 24th on August 10, 1821.

The 36 degrees 30 minutes line held until the Kansas-Nebraska Act of May 30, 1854 effectively repealed it by introducing popular sovereignty, allowing settlers to decide on slavery regardless of latitude. The Supreme Court ruled the original Missouri Compromise unconstitutional retroactively in Dred Scott v. Sandford on March 6, 1857. Thomas Jefferson called the Missouri crisis "a fire bell in the night" warning of disunion. The compromise managed to defer the slavery crisis for a generation, but it could not resolve the underlying contradiction.