After the Louisiana Purchase, the size of the U.S. doubled. This increased the opportunity for settlers to move west, but the issue of slavery became bigger in Congress. At the time, the number of slave states was equal to the number of free states. Therefore, any new state would make the balance unequal.

Missouri Territory applied for statehood in 1819. At that time it was a farming community, meaning there were more the 10,000 slaves legally owned.

Slaves were considered property, so outlawing of slavery would have affected its economy.

Senators from free states did not agree with the idea of legal slavery, and the southern senators felt that slave labor was important for farming. The Congress was at a standstill for an entire year over the admittance of Missouri.

In 1820, the Maine Territory applied for statehood as a  free state. This meant with the admittance of Missouri as a slave state, would make it equal again.