The Missouri Compromise

By 1819 the Union had eleven free states and eleven slave states, so the Senate was split evenly, 22 to 22. That balance was not an accident. The Senate gives every state two seats no matter its population, so it was the one chamber where slave states could not be outvoted by the faster-growing free population of the North. When Missouri asked to join as a slave state, it would tip that balance for the first time, and Northern congressmen saw a precedent they could not accept as slavery pushed west with the cotton frontier.

Henry Clay's compromise, passed in 1820, solved the immediate problem by pairing Missouri's admission as a slave state with Maine's admission as a free state, keeping the Senate at 12 and 12. It also drew a line across the rest of the Louisiana Purchase at 36 degrees 30 minutes latitude, Missouri's own southern border extended west. North of that line, slavery was banned, which covered most of the remaining unorganized territory. South of it, slavery was permitted, though almost all of that land already lay within Missouri or the future Arkansas Territory, so little new slave territory actually opened.

The compromise held for a generation because both sections treated the line as a fixed bargain rather than a political football. That changed in 1854, when the Kansas-Nebraska Act let settlers in Kansas and Nebraska decide slavery for themselves by popular sovereignty, even though both territories sat north of 36 30. That law effectively repealed the Missouri Compromise's ban, triggered violent conflict known as Bleeding Kansas, and showed that a line on a map only holds as long as both sides still agree to honor it.