The impact of the Missouri Compromise was that it maintained the balance of slavery and anti-slavery states and postponed the eruption of the Civil War. It was also the first time Congress became involved in the regulation of slavery.
In 1819, Missouri applied for admission to the union as a slave state. At the time, slave states and anti-slave states were evenly balanced, and neither side wanted to cede advantage to the other. Southern slave states argued that new states should be free to choose slavery, while northern anti-slave states insisted that Congress had the power to ban slavery. The Missouri Compromise had two parts. Firstly, Missouri was admitted to the union as a slave state, but Maine was admitted as a free state. Also, an imaginary line across the territory of the Louisiana Purchase separated free and slave regions.
The Missouri Compromise remained law until 1854, when it was repealed by the Kansas-Nebraska Act that allowed the populace of these two new states to locally choose their status of slave or anti-slave. Three years later, the U.S. Supreme Court declared the Missouri Compromise unconstitutional. The slavery issue continued to provoke controversy until it erupted in the American Civil War, fought from 1861 to 1865, during which over 600,000 soldiers were killed.