The Missouri Compromise was a congressional act passed in 1820 that attempted to balance the number of free and slave states. It prohibited slavery north of the 36’30” parallel (which runs near St. Louis). The compromise was meant to appease both northerners and southerners, but it did not work.
In 1819, Missouri applied for statehood as a slave state. Northerners objected because they wanted all new states entering into the union as free states. However, southerners felt that they should have equal representation in Congress with their slaves counted as three-fifths of a person (the Constitution counted slaves as “three-fifths” of a person for purposes of apportionment for direct taxes). Many northerners did not want slavery expanded into Missouri because they feared it would lead to increased competition between northern and southern farmers for land and jobs; however, others thought that restricting slavery in any way was wrong morally or religiously.
Prior to 1820, there were 15 free states and 15 slave states in the Union. The result was an equal number of representatives in Congress — 60 each for free and slave states. However, by 1820, there were 24 free states and 12 slave states, which resulted in an imbalance that favored free states in Congress. In order to resolve this imbalance, Southerners pushed for Missouri’s admission as a slave state to maintain their representation in Congress and keep things equal between North and South.
This led to greater tensions between North and South and lead up to the Civil War.