Article 1 section 8 of the Constitution:
The Congress shall have Power:
"To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water."
"To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years"
"To provide and maintain a Navy"
"To make Rules for the Government and Regulation of the land and naval Forces."
"To provide for organizing, arming, and disciplining the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress"
It is pretty clear what POWER the Congress has if it decides to get its head out of its ass and actually use it.
It can close Gitmo
It can end the war in Iraq
It can decide pretty much the whole war business. The President is basically just another GI in America's armed forces.