South wanted to keep slaves, the North were growing openly against it and was pushing to make slavery illegal, so the South wanted to secede from the US to keep their slaves
Lincoln didn't want the states to divide over it, so the South declared war on Lincoln for the right to secede so they could keep their slaves
Then Lincoln says his goal is not only to keep the states united, but to ban slavery across all the states
South tries to fight but loses
Thus, slavery ends in the US