Missouri Compromise, an 1820 agreement that settled the slavery question for about 30 years. The compromise provided for the admission of Missouri into the Union as a slave state but prohibited slavery in the rest of the northern Louisiana Purchase territory. It went into effect through an act of Congress signed by President James Monroe, March 6, 1820.

Before the Missouri Compromise was adopted, the issue of slavery extension was thoroughly debated for a year in Congress and throughout the nation. Speeches and writings revealed the sectional differences that were eventually to tear the Union asunder. Thomas Jefferson wrote: “This momentous question, like a fire bell in the night, awakened and filled me with terror.” John Quincy Adams called it “a mere preamble—a title-page to a great, tragic volume.”

The Problem

In February, 1819, the House of Representatives considered a bill authorizing the territory of Missouri to frame a constitution (the first step toward statehood). In the past, territories south of the Ohio River and the Mason and Dixon Line had been automatically made slave states. Most of Missouri is north of that line but many of its citizens were slaveholders.

A bill to permit slavery in Missouri on a temporary basis, but prohibiting further entry of slaves, passed the House but was rejected by the Senate. The matter was left unsettled until the next session of Congress. Meanwhile, Alabama had been admitted as a slave state, making the number of slave and free states 11 each. This added another problem: if Missouri were admitted, the balance in the Senate (where each state had two votes) would be upset.

The Solution

The House passed a bill, March 1, 1820, requiring Missouri to be a free state. The Senate dropped the antislavery provision and added an amendment proposed by Senator J. B. Thomas of Illinois. The amendment provided that Missouri would be a slave state, but that slavery would be prohibited in other parts of the Louisiana territory north of 36° 30' (Missouri's southern border). This amendment was the heart of the compromise; it was made possible because Maine had applied for statehood as a free state and the admission of both Missouri and Maine would maintain the balance between free and slave states.

Largely through the efforts of Henry Clay, then speaker of the House, the amendment was accepted by the House. Missouri was authorized to submit a proslavery constitution and Maine was admitted to the Union.

The Missouri constitution was presented to Congress for approval in 1821. It included a paragraph requiring the legislature to prevent the immigration of free blacks into the state. The antislavery faction in Congress objected to this provision and a compromise bill—often called the Second Missouri Compromise—was passed on March 2, 1821. This measure forbade Missouri to limit the rights guaranteed to all citizens by the federal government, meaning it could not ban the entry of free blacks into the state. Missouri was admitted on August 10, 1821.

Later History

An unsuccessful attempt was made in 1848 to broaden the Missouri Compromise to include the territories gained in the Mexican War, by extending the line of 36° 30' to the Pacific Ocean. The compromise was repealed by the Kansas-Nebraska Act (1854). In the Dred Scott Decision (1857), the U.S. Supreme Court ruled that Congress did not have the authority to abolish slavery in the territories, and that therefore the Missouri Compromise had been unconstitutional. Ironically, Missouri did not become a strong slave state, and did not secede from the Union during the Civil War.