The Missouri Compromise
Missouri applied for statehood on December 18, 1818. This created a problem because the Northern states refused to allow another slave state to join the Union.
In 1819, Maine applied for statehood. Then a compromise developed: Maine could join as a free state to balance out Missouri joining as a slave state.
By 1820, the compromise had been realized. First, Missouri and Maine would be admitted to the Union, one as slave and one as free. In addition, the remaining territories would be divided into free and slave at the 36°30' parallel. This compromise became viewed as sacred by both sides.
Helped cause civil war because it was the first time U.S. delt with slavery