When Missouri wanted to enter the Union as a slave state, the North became worried because this were to tip the balance. There were an equal number of slave states and free states, but now Missouri wanted to be a slave state.
This lead to the Missouri Compromise of 1820, which kept the senators between the North and South equal. This allowed Missouri to become a slave state, and Maine to become a free state.
Although this solved some issues for now, it deepened the tensions between the North and the South.
