The North was the first to take hostile action. That is quite clear. That South Carolina responded doesn’t change that fact.
Unilateral secession itself was a hostile act. Compound that with the theft of federal property and the assault on a federal fort and you have an act of war.
Double-down by declaring war as the south did on May 6, 1861 and there is no doubt who the aggressor was. That the United States responded doesn’t change that fact.