The Confederate States of America committed an act of War against the United States, when they fired on Fort Sumter.
That act and Davis’s authorization of letters of Marque and Reprisal against U.S. shipping were all that the Lincoln administration needed to justify the use of armed force to put down the “rebellion”. Jeff Davis gave Abe Lincoln all the ammunition he needed to make war on the Confederate States.
The Union had already launched their strike. It never landed because the Confederates found out about it, and took Sumter before the Union could deliver the first blow.
But the Union strike force was on the way when it all happened. They just don't teach you the parts of history that make them look like the bad guys.
Pea Ridge has posted excellent contemporary letters regarding Lincoln's secret strike force which was under way when the events at Sumter made their mission unnecessary.
I will also point out that under the precedent of the Revolutionary war, the Military assets at Lexington and Concord belonged to the colonists, not the British. The Union should have relinquished control of that fortress which belonged to South Carolina before the Union ever existed.
No nation on earth would accept a foreign controlled fortress in one of their Major cities.