“That was in dispute with many believing that there never was a “CSA” as a legal entity.”
Perhaps, but it’s not in dispute that the state of South Carolina considered it a legal entity. Doesn’t take a genius to realize if you insisted on stationed troops in their borders, THEY were going to view it as an act of war. This is one of those things where you don’t need a mutually agreed upon perception to make something a reality; one side’s perception is all it takes.
So they declared themselves seceded from the union even though they knew it was a provocative act. The soldiers that were stationed in the area - who were Americans BTW - instantly became "the enemy" (even if they were southerners). The townspeople and local militia immediately became hostile and belligerent. Anderson, not wishing to ratchet up tensions decided to retreat to Sumter until he had clear orders to withdraw but Beauregard insisted that he quit the fort. Ultimately the southerners fired on the fort until the union forces surrendered.
The southerners deliberately instigated the hostilities which inevitably led to the war. THAT was an act of war.