The Missouri Compromise was important as it strengthened the Union for over 30 years, despite there being concerns regarding establishing new states as slave states. In the events that led up to the compromise, the North was concerned about establishing new states as slave states, whereas the South was concerned about letting Congress infringe on each state's rights to make its own slavery laws.
Prior to the 1820 Missouri Compromise, Missouri wished to join the Union as a slave state, but Congress was concerned about the expansion of slavery across the U.S. When Missouri made the request to join as a slave state, there were 22 member states. As there was an equal divide between slave and free states, Congress grew concerned that adding another slave state would upset the balance. As a compromise, it agreed to admit Missouri, but also established Maine as a free state, and drew an imaginary border across Louisiana that would divide it into slave and free areas.
Although the Compromise temporarily established calm over the issue, both the North and the South had concerns. While the North believed this encouraged the expansion of slavery throughout the union, the South saw the decision as allowing Congress to disrupt state sovereignty. The Compromise was later revoked in 1854 with the Kansas-Nebraska Act.