Firing on Ft. Sumter, etc. was an act if war. But not just any war. The North was by no means empowered to conquer the Confederacy and reintegrate secessionist stated because they stole federal property. Taking back the forts and demanding retribution and reparations, that’s what Sumter justified, if that. Not total war, unconditional surrender, and occupation which in a sense persists to this day.
As for the notion that the South invaded the North, lol. Who are you kidding? Even if we take your point about Bull Run on its face, that was only after the blockade of Southern ports, which SCOTUS later pinpointed as the start of the war, and Lincoln’s (unconstitutional) call for volunteers, not to defend Washington but obviously to crush the “insurrection.”
Okay, well then let’s us just say it was the war to keep slavery in the South war. The South wanted to keep slaves and they attacked the North because of it. So the issue was the South wanted to keep slaves.