Missouri was the territory whose request to become a state sparked a battle over slavery in 1819. At the time, there were 22 states in the country, and they were equally divided - 11 were slave, and 11 were free. Now, both Maine and Missouri wanted to enter as states, and both wanted to be free states, however, in order to maintain balance, Missouri was admitted as a slave state, while Maine was a free state, which sparked many battles.
