Answer:
They expanded westward, therefore colonizing more of America and making new states. But, as new states were added, they have to decide which states are going to be slave states and which are going to be free states, and the Northerners really didn't want slavery, even though they still treat the African Americans worse there than when they were slaves. And then, the dispute got worse and worse, eventually starting a war within the country which left them very vulnerable to attack from other countries, but it never happened.