No, South Carolina was within the Confederate States, a internationally recognized legal government upholding its sovereignty. All of its acts to make the garrison of Fort Sumter to evacuate were legal by international laws. The laws of the United States did not apply since it did not have the power to enforce its justice over South Carolina. Due to the fact the Queen of England, head of the powers of international law, recognized the Confederacy as a co-belligerent in a war, its actions, as long as they followed internationl law, were legal.
This fact made prosecuting the leaders of the Confederacy very difficult.