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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.

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.

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.

In 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 question...like 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.”

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

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.

​Source

​History.com - Missouri Compromise