Missouri Compromise, package of measures adopted by the U.S. Congress in 1820–21, to resolve issues relating to the extension of slavery. At the time of Missouri's first petition to become a state (1819), there were 11 free and 11 slave states in the Union. Missouri's addition would have changed the balance of power in the U.S. Senate and reopened the bitterly contested issue of whether slavery should be permitted to spread in the United States. Action on Missouri's petition was delayed until Maine requested admission as a free state, and both were admitted. The compromise also barred slavery from being extended to the rest of the territory of the Louisiana Purchase north of 36°30' latitude. The compromise was repealed in 1854 by the Kansas-Nebraska Act.