The 1819 application for statehood by the Missouri Territory sparked a bitter debate in Congress over the issue of slavery in the new territories that had been created as a result of the Louisiana Purchase of 1803. Concerned that the South would have a representational advantage, Congressman James Tallmadge of New York introduced an amendment that would prohibit any further growth of slavery in Missouri, and would eventually set the children of Missouri's slaves free.

Despite "the difficulties and the dangers of having free blacks intermingling with slaves," Tallmadge declared, "I know the will of my constituents, and regardless of consequences, I will avow it; as their representative, I will proclaim their hatred to slavery in every shape." The bill passed in the House but failed to pass the Senate.

The issue was resolved with a two-part compromise. The northern part of Massachusetts became Maine and was admitted to the Union as a free state at the same time that Missouri was admitted as a slave state, thereby maintaining a balance of 12 slave and 12 free states.

In addition, an imaginary line was drawn at 36 degrees 30 minutes north latitude, and any portions of the Louisiana Territory lying north of the compromise line would be free; however, the act provided that fugitive slaves "escaping into any... state or territory of the United States...may be lawfully reclaimed and conveyed to the person claiming his or her labour or service" -- and even in the free territories, "slavery and involuntary servitude ... in the punishment of crimes" was not prohibited.