The Missouri Compromise of 1820 was a law that tried to address growing sectional tensions over the issue of slavery. By passing the law, which President James Monroe signed, the U.S. Congress admitted Missouri to the Union as a state that allowed slavery, and Maine as a free state. It also banned slavery from the remaining Louisiana Purchase lands located north of the 36º 30’ parallel (the southern border of Missouri). The Missouri Compromise would remain in force for just over 30 years before it was repealed by the Kansas-Nebraska Act of 1854. In 1857, the Supreme Court ruled in the Dred Scott decision that the compromise was unconstitutional, setting the stage for the Civil War.

Pro- and Anti-Slavery Factions in Congress

When the Missouri Territory first applied for statehood in 1818, it was clear that many in the territory wanted to allow slavery in the new state. Part of the more than 800,000 square miles bought from France in the Louisiana Purchase of 1803, Missouri was known as the Louisiana Territory until 1812, when it was renamed to avoid confusion with the newly admitted state of Louisiana.

Missouri’s bid to become the first state west of the Mississippi River, and to allow slavery within its borders, set off a bitter debate in a Congress that was—like the nation itself—already divided into pro- and anti-slavery factions.

In the North, where abolitionist sentiment was growing, many people opposed the extension of the institution of slavery into new territory, and worried that adding Missouri as a slave state would upset the balance that currently existed between slave and free states in the Union. Pro-slavery Southerners, meanwhile, argued that new states, like the original 13, should be given the freedom to choose whether to permit slavery or not.

During the debate, Rep. James Tallmadge of New York proposed an amendment to the statehood bill that would have eventually ended slavery in Missouri and set free the existing enslaved workers living there. The amended bill passed narrowly in the House of Representatives, where Northerners held a slight edge. But in the Senate, where free and slave states had exactly the same number of senators, the pro-slavery faction managed to strike out Tallmadge’s amendment, and the House refused to pass the bill without it.

Maine and Missouri: A Two-Part Compromise

After this stalemate, Missouri renewed its application for statehood in late 1819. This time, Speaker of the House Henry Clay proposed that Congress admit Missouri to the Union as a state that allowed slavery, but at the same time admit Maine (which at the time was part of Massachusetts) as a free state.

In February 1820, the Senate added a second part to the joint statehood bill: With the exception of Missouri, slavery would be banned in all of the former Louisiana Purchase lands north of the 36º 30’ latitude, which ran along Missouri’s southern border.

On March 3, 1820, the House passed the Senate version of the bill, and President James Monroe signed it into law four days later. The following month, the former President Thomas Jefferson wrote to a friend that the “Missouri a fire bell in the night, awakened and filled me with terror. I considered it at once as the knell of the Union. It is hushed indeed for the moment. But this is a reprieve only, not a final sentence.”

Shortcomings of the Missouri Compromise

Though the Missouri Compromise managed to keep the peace—for the moment—it failed to resolve the pressing question of slavery and its place in the nation’s future. Southerners who opposed the Missouri Compromise did so because it set a precedent for Congress to make laws concerning slavery, while Northerners disliked the law because it meant slavery was expanding into new territory.

In the decades after 1820, as westward expansion continued, and more of the Louisiana Purchase lands were organized as territories, the question of slavery’s extension continued to divide the nation. The Compromise of 1850, which admitted California to the Union as a free state, required California to send one pro-slavery senator to maintain the balance of power in the Senate.

In 1854, during the organization of Kansas and Nebraska Territories, Senator Stephen Douglas of Illinois spearheaded the Kansas-Nebraska Act, which mandated that the settlers of each territory should decide the issue of slavery for themselves, a principle known as popular sovereignty.

The controversial law effectively repealed the Missouri Compromise by allowing slavery north of the 36º 30’ parallel. Passage of the Kansas-Nebraska Act sparked violence between pro- and anti-slavery settlers in “Bleeding Kansas,” delaying Kansas’ admission to the Union. Opposition to the act led to the formation of the Republican Party, and the emergence to national prominence of Douglas’s Illinois rival, a formerly obscure lawyer named Abraham Lincoln.

Bitter controversy also surrounded the U.S. Supreme Court decision in Dred Scott v. Sandford, which ruled that the Missouri Compromise was unconstitutional. According to Chief Justice Roger B. Taney and six other justices, Congress had no power to prohibit slavery in the territories, as the Fifth Amendment guaranteed owners of the enslaved could not be deprived of their property without due process of law. The 14th Amendment, passed in 1865 after the conclusion of the Civil War, would later overturn major parts of the Dred Scott decision.