The Missouri Compromise, an agreement between pro-slavery and anti-slavery factions, was a federal statute passed into law in the United States in 1820. It prohibited slavery in what was the Louisiana Territory north of parallel latitude 36°30', but allowed Missouri to be admitted to the Union as a slave state.
Missouri's admission into the Union sparked controversy because northerners generally believed that Congress should prohibit slavery in new states. Southerners generally believed states had the right to choose for themselves. When the House and Senate passed different bills that threatened to leave the situation at a standstill, the Missouri Compromise was formulated by Henry Clay. The compromise also admitted Maine, which had formerly been part of Massachusetts, as a free state.
Though many northerners and southerners were unhappy with the compromise, it helped stabilize the union for over 30 years. The compromise was eventually repealed by the Kansas-Nebraska Act of 1854. Both Kansas and Nebraska were north of the compromise line, but the new act established "popular sovereignty," in which each territory would make its own choice regarding slavery. Though the Kansas-Nebraska Act essentially invalidated the Missouri Compromise, the Supreme Court officially ruled that the Missouri Compromise was unconstitutional in Dred Scott v. Sanford, a decision handed down in 1857.