That is a good post. The immediate cause of the war was over the Constitutional principle of secession. The U.S. government refused to recognize the right of the southern states to secede from the Union and the CSA government asserted that right by seizing federal property. However, the issue of slavery was an important contributing factor that eventually led to the war.The war was over secession. Secession was over slavery. The Southern Slave States seceded to protect slavery. They then took federal property and fired on federal soldiers to force their secession. In reaction the government went to war to suppress the rebellion.
No slavery, no secession. No secession, no war.