For decades, both northern states and southern states had threatened secession and dissolution of the Union over the question of where slavery was to be permitted. At issue was power. Both sides sought to limit the governing power of the other by maintaining a balance of membership in Congress. This meant ensuring that admission of a new state where slavery was outlawed was matched by a state permitting slavery. For example, at the same time that Missouri entered the Union as a slave state, Maine entered the Union as a free state.