The Missouri Compromise

In 1819 the Union had eleven free states and eleven slave states, giving each section equal votes in the Senate even as the North’s faster-growing population gave it a majority in the House. The Louisiana Purchase had added an immense territory with no settled rule on slavery. Missouri, settled largely by Southerners who brought enslaved people with them, applied for statehood.

In February 1819, Representative James Tallmadge of New York proposed amending Missouri’s statehood bill to prohibit bringing more enslaved people into the state and to provide for the gradual emancipation of those already there. After fierce debate, the House adopted the amendment by a straight sectional vote; the Senate defeated it. Northerners attacked the expansion of slavery, and Southerners insisted Congress had no power to restrict it in a new state.

The deadlock broke when Maine, long part of Massachusetts, sought statehood. Congress paired the two admissions: Missouri would enter as a slave state and Maine as a free state, preserving the balance in the Senate. Congress also prohibited slavery in the rest of the Louisiana Territory north of 36°30′, while the land south of the line — organized as Arkansas Territory — stayed open to slavery. A conference committee reported the compromise on March 1, 1820, and it became law. A second dispute erupted when Missouri submitted a constitution denying free Black people the right to live in the state; it was resolved with a further proviso, and Missouri was admitted in 1821.

The compromise calmed the immediate crisis and kept the two parties of the era from splitting along sectional lines. But it established that the balance of slave and free states was a central political question, to be renegotiated with every new territory.

For a generation the 36°30′ line kept an uneasy peace. It collapsed when the Mexican-American War added new lands, and Congress repealed it outright in the Kansas-Nebraska Act of 1854. In 1857 the Supreme Court declared in Dred Scott that Congress had never had the power to ban slavery in the territories, making the compromise retroactively unconstitutional. The question it had postponed — whether slavery would expand west — led directly to the Civil War.