Except the south fired the first shot to keep the north from reinforcing Fort Sumter, as US Army fort in South Carolina. SC and the CSA wanted to keep US Army installations as their own. For free. But that was in contravention of the Constitutional clause dealing with federal property.

If slavery caused secession then it caused the war too because the only way to avoid the fight was to let the south leave… Over slavery! And also agree to their demand to get to keep all federal property in their states for free. Wasn’t going to happen and they knew it. It’s like claiming you didn’t hit your brother, you just walled toward him throwing punches and he didn’t move.