The US Congress was constantly trying to keep a balance of free and slave states added to the US between 1816 and 1821. A perfect example of this balancing act was the Missouri Compromise. The Missouri Compromise (1820) was a law that allowed Missouri to enter as a slave state and Maine to enter as a free state.
The goal of these types of laws was to keep a balance of power in Congress between slave and free states. Without this balance, one group could overpower another. For example if the free states had more representatives in Congress, they might try to make a law to get rid of slavery. The US continues this balancing act for decades but fails to solve the issue of slavery. This leads to the beginning of the Civil War.