Of course, during Buchanan’s term, Confederate forces in Charleston fired on a ship flying the United States flag and did nothing about it. To my knowledge, firing on an unarmed ship on a peaceful mission is an act of war.
When the Confederates fired on Fort Sumter, they committed an act of war. Lincoln did not call for 75,000 volunteers because the Southern States had seceded, he did so because they launched an armed rebellion against the United States.
The ship, Star of the West, originally civilian, had just been pressed into military service to avoid alarming the Charleston citizenry, and outfitted for battle. It carried 200 troops armed with muskets, war munitions, arms, and supplies for one year.
It was a military expedition force designed to enter a non-Union port and land against local authority.
The Governor of South Carolina had just announced that any attempted improvement in military status by the Union would constitute an act of war.
There is your act of war.
The Union ship's company and 200 troops should have been appreciative that the Charleston citizens and Citadel students let them escape unharmed.