[Cacophonous]

Article 1, Section 8:

The Congress shall have Power To...declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water

This means that it is Congress that can declare war, not the executive. If the President wants to declare war, he is obligated to get Congress to do it. He (the President) then executes the war.

[CapandBall]

...or not. The constitution does not stipulate that war must be "declared" in order for the US to wage war.