The Missouri Compromise was a federal legislation passed by the United States Congress in 1820 to maintain the balance of power between slave and free states in Congress. The compromise admitted Missouri as a slave state and Maine as a free state at the same time, so as not to upset the balance between slave and free states in the Senate. The compromise also proposed that slavery be prohibited above the 36º 30 latitude line in the remainder of the Louisiana Territory. This provision held for 34 years, until it was repealed by the Kansas-Nebraska Act of 1854. In 1857, the Supreme Court ruled that the Missouri Compromise was unconstitutional in its Dred Scott v. Sandford decision. The Missouri Compromise was very controversial, and many worried that the country had become lawfully divided along sectarian lines. The compromise both delayed the Civil War and sowed its seeds.

In summary, the Missouri Compromise was a federal legislation passed in 1820 to maintain the balance of power between slave and free states in Congress. It admitted Missouri as a slave state and Maine as a free state at the same time, and prohibited slavery above the 36º 30 latitude line in the remainder of the Louisiana Territory. The compromise was repealed by the Kansas-Nebraska Act of 1854 and declared unconstitutional in the Dred Scott v. Sandford decision in 1857.