Missouri Compromise
1820

In order to maintain the balance of free and slave states, Maine was admitted to the Union as a free state and Missouri as a slave state. It also decreed that all states admitted north of the 36 30" line would be free, and states admitted south of the line would allow slavery.
The Missouri Compromise only delayed the inevitable confrontation that would be necessary when a decision was finally made. The 36 30" law was discarded when California requested statehood, because it was both above and below the line, so the Compromise of 1850 was implemented instead.