States' rights
While many people believe that the Civil War was about states' rights, not slavery, the Southern states were in favor of states' rights for themselves, not for all states. They did not favor states' rights for new states joining the Union on the question of slavery. They wanted the federal government to decide that all news states would allow slavery. That can be seen in the Fugitive Slave Act of 1793, the Missouri Compromise of 1820, and the 1850 Compromise. All  oth themwere designed to force the Northern states to accept slavery in the territories.