According to Bruce Ackerman and Oona Hathaway, "The Bush administration's infatuation with presidential power has finally pushed the country over a constitutional precipice. As of New Year's Day, ongoing combat in Iraq is illegal under US law."
Ackerman and Hathaway note that in 2002, Congress authorized a two-purpose invasion of Iraq: first it was to "defend the national security of the US from the threat posed by Iraq," and secondly, to "enforce all relevant UN Security Council resolutions."
Any supposed threat from Iraq is long gone. "Our continuing intervention has been based on the second clause of Congress' grant of war-making power. Coalition troops have been acting under a series of Security Council resolutions athorizing the continuing occupation of Iraq."
Bush, however, let the UN mandate expire as of December 31. "At precisely one second after midnight, Congress' authorization of the war expired along with this mandate."
So, why are coalition troops still in Iraq? Ackerman and Hathaway report that "Bush is trying to fill the legal vacuum with the new Status of Forces Agreement (SOFA) he signed with the Iraqis. But the president's agreement is unconstitutional, since it lacks the approval of Congress. Bush even refused to allow Congress access to the terms of the deal." [My emphasis]
What to do?
Read all of what Ackerman and Hathaway have to say here.