Lincoln had acted unilaterally because he did not want to ask Congress for a declaration of war, believing that such an act would give the Confederacy recognition as an independent nation. Nations do not blockade their own ports, however; they simply close them. Lincoln did not want to do that either because he feared what might happen when foreign nations like Great Britain attempted to enter such ports.
In 1863, the Supreme Court ruled on the legality of Lincoln’s activities in The Prize Cases 67 U.S. 535. The Court considered the question of whether the president could take action on his own when the nation came under attack or did he have to wait for Congress to ratify his action before he could take it. The Court majority ruled that, based on Article II of the Constitution, the president had the constitutional authority to act as he had. As Constitutional historian Daniel Farber has argued, Lincoln complied with the War Powers Resolution, one hundred years before it was passed.