Um, so it was the North that fired on Ft. Sumter?
It was the North which refused to relinquish the fortification to the proper authorities. That encroachment was the first act of invasion.
And by the way — that was one State. If the Union felt so irrevocably aggrieved by that one action that it was somehow compelled irretrievably into war. Then its war could only legitimately have been against South Carolina.
But I imagine everyone in hindsight can see that the Sumpter incident could and should have been quickly and peacefully resolved once the shooting there stopped. There was no legitimate call for a Northern invasion predicated upon that one incident.