The civil war was about states' rights.

States' rights to not send escaped slaves back to their home states where slavery is legal. The South wanted to force the North to give return escaped slaves. The North said no, if they come here they're free. So the South started a war about it, because they didn't want the states to have the right to make their own decisions about slavery. The South wouldn't put up with any state supporting slaves who relocated to within its borders.