If a war be made by invasion of a foreign nation, the President is not only authorized but bound to resist force by force. He does not initiate the war, but is bound to accept the challenge without waiting for any special legislative authority. And whether the hostile party be a foreign invader or States organized in rebellion, it is nonetheless a war although the declaration of it be "unilateral."
Justice Grier, Prize Cases, 67 U.S. 635 (1862)
The Supreme Court held that it was a "war". Also Lincoln himself stated that his actions were unconstitutional:
"I felt that measures, otherwise unconstitutional, might become lawful, by becoming indispensable to the preservation of the constitution, through the preservation of the nation."
Abraham Lincoln, "To Albert G. Hodges", 4 Apr 1864, Collected Works Of Abraham Lincoln, (Roy P. Basler, Ed.), Vol VII, p. 281.