The Missouri Compromise was criticized by many southern states because it established the principle that Congress could make laws regarding slavery; northerners, on the other hand, condemned it for acquiescing in the expansion of slavery (though only south of the compromise line).
The Missouri Compromise was an effort by Congress to defuse the sectional and political rivalries triggered by the request of Missouri late in 1819 for admission as a state in which slavery would be permitted.
At the time, the United States contained twenty-two states, evenly divided between slave and free. Admission of Missouri as a slave state would upset that balance; it would also set a precedent for congressional acquiescence in the expansion of slavery.
In the years leading up to the Missouri Compromise of 1820, tensions began to rise between pro-slavery and anti-slavery factions within the U.S. Congress and across the country. They reached a boiling point after Missouri’s 1819 request for admission to the Union as a slave state, which threatened to upset the delicate balance between slave states and free states.
To keep the peace, Congress orchestrated a two-part compromise, granting Missouri’s request but also admitting Maine as a free state. It also passed an amendment that drew an imaginary line across the former Louisiana Territory, establishing a boundary between free and slave regions that remained the law of the land until it was negated by the Kansas-Nebraska Act of 1854.
Three years later, the Supreme Court in the Dred Scott case declared the Missouri Compromise unconstitutional, on the ground that Congress was prohibited by the Fifth Amendment from depriving individuals of private property without due process of law.