Which territories request to become a state sparked a battle over slavery in 1819?

The request for Missouri to become a state in 1819 sparked a significant battle over slavery, leading to the Missouri Compromise. As Missouri sought admission as a slave state, it raised tensions regarding the balance of free and slave states in the Union. This conflict highlighted the deep divisions over slavery in the United States, ultimately resulting in the compromise that admitted Missouri as a slave state while Maine entered as a free state, maintaining the balance.