The Missouri Compromise was an act designed to create a compromise in the increasingly divisive U.S. Senate, but it eventually led to more tension about slavery and was a contributing factor in the American Civil War. The act allowed Missouri to join the nation as a slave state, Maine to join as a free state, and banned slavery in any new states north of Missouri. It was later deemed unconstitutional.
In 1819, Missouri petitioned to join the United States. While new states were added to the union several times before, Missouri's entrance into the union sparked a heated controversy in the Senate. In 1819, before Missouri's entrance, the United States was composed of 11 free states and 11 slave states. The nation, and thus the Senate, was balanced between slave and free states. Missouri's entrance into the union as a slave state threatened to upset that balance leading to a long and bitter debate. Eventually, congressman Henry Clay helped to craft a solution to the potential imbalance. Under the Missouri Compromise, Missouri would be allowed to join the United States as a slave state if Maine was added as a free state. This ensured the balance in the Senate would continue. However, the act also banned slavery in any new states north of Missouri. While the act passed, it was eventually overturned by the Kansas-Nebraska act and ruled unconstitutional. The Missouri Compromise and the subsequent ruling led to growing controversy over state's rights versus federal rights and played a part in the American Civil War.