What was the main issue that divided the nation and led to the Civil War?

The Mexican American war led to the gain of territories which flared up the debates over slavery. The Missouri compromise made it illegal to have slavery, basically making it illegal to have slaves in the territories that was gained in the Mexican American war. A lot of Southern Secession was beginning to happen after the election of Lincoln meaning southern states started to leave the union, southern states depend on farming and plantations meaning they need slave labor for that, and the north was further industrialized so they didn’t need slave labor they feared that they would be forced to become free states as well