Uh, I'm sorry...where exactly were the "president's war powers" enumerated in the US Constitution?
Article II, Section 2
"Clause 1: The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States;..."
President Lincoln wrote:
"You dislike the emancipation proclamation; and perhaps, would have it retracted. You say it is unconstitutional--I think differently. I think the Constitution invests the commander in chief with the law of war, in time of war. The most that can be said, if so much, is, that slaves are property. Is there--has there ever been--any question that by the law of war, property, both of enemies and friends, may be taken when needed? And is it not needed whenever taking it helps us, or hurts the enemy?"
Whatever you think of it, it is a commonly held interpretation.
Note also that the Militia Act of 1795 leaves when to call out the militia to the judgement of the president.
Walt