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.
What “strike” was that. Lincoln had told the Governor of SC that he intended to resupply Sumter via row boat. That nothing but provisions, no ammunition, no additional troops would be provided to the fort. The ships off the coast were there to do just that.