Southern states did not levy war against them (the states)
The first cannon ball to bounce of the masonry of Fort Sumter constituted Levying war against the United States.
Davis would have fired on Sumter if Lincoln had sent a troop of girl scouts in open canoes filled with food to resupply Sumter. He was not going to allow the fort to be resupplied under any circumstance.
An aggressor is one who invades the land of another - not one who fires to drive an invader away.