In testimony before the Senate Judicary Committee, Bradford Berenson, who served as Associate Counsel to President Bush from 2001-03, conceded that Congress has a broad power to limit the Commander-in-Chief:
In my view, the questions whether Congress can require a complete cessation of hostilities, impose a troop ceiling, or limit the geographic scope of warfare in the Middle East falls into the realm of shared powers not specifically addressed by the text of the Constitution. Thus, as an abstract matter, Congress has, through the Spending Power and the Necessary and Proper Clause, sufficient authority to enact facially and presumptively constitutional legislation restricting the Executive’s freedom of action by defining the broad contours of permissible military engagement.
Berenson's view, that Congress may forbid the President from escalating the Iraq War or even order the war to an end, echoes the view of Professor Neil Kinkopf, who published The Congress as Surge Protector with ACS:
The Supreme Court has been clear and unambiguous. When Congress, acting in the vast areas of overlapping power, tells the President “no,” the President must comply. Thus, Congress may limit the scope of the present Iraq War by either of two mechanisms. First, it may directly define limits on the scope of that war—and forbid the President from exceeding these limits—such as by imposing a ceiling on the number of troops assigned to that conflict. Second, it may achieve the same objective by enacting appropriations riders that limit the use of appropriated funds. Indeed, the reason that the Constitution limits military appropriations to two years is to prevent Congress from abdicating its responsibility to oversee ongoing military engagements.
In related news, two Senators recently introduced separate bills which would end the Iraq War. Senator Russ Feingold's (D-WI) bill would cut off all funding for the Iraq War six months after its enactment. Senator Barack Obama's (D-IL) bill would order redeployment of U.S. troops out of Iraq begining in May of 2007, with the last servicemembers leaving by March 31, 2008.