I personally think the war was about whether we would be a nation with all power in Washington of some power in Washington and most in local and the states. Slavery was used to inflame the population but mostly not until the war was over.
Lincoln made it plain that he was fighting to save the Union. and the Confederates to save their way of life which included slavery but not for much longer.
650,000 Americans lost their lives so Lincoln could be president of the whole country. Two thirds of the deaths were Yankees but they had them to lose as they outnumbered the South more than 4 to 1 plus outnumbered them far more in the munitions of war. Also probably a million or more were maimed for life.
That cost was way too much to pay for what would eventually have occurred anyway if you think it was about slavery. Modern machinery was rapidly making slavery an outdated operation.
Exactly. States rights.