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.

In authorizing an invasion in 2002, Congress did not give President Bush a blank check. It explicitly limited the use of force to two purposes: to “defend the national security of the US from the threat posed by Iraq” and “enforce all relevant UN Security Council resolutions.”

At precisely one second after midnight, Congress’ authorization of the war expired… The question is how President Obama should respond to the legal catastrophe that Bush has left as his Iraqi legacy.

Five years after the fall of Saddam Hussein, the government of Iraq no longer poses a threat. 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 authorizing the continuing occupation of Iraq. But this year, Bush allowed the UN mandate to expire on December 31 without requesting a renewal. At precisely one second after midnight, Congress’ authorization of the war expired along with this mandate.

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. By contrast, Prime Minister al-Maliki followed his constitution and submitted the agreement for parliamentary approval. While the Iraqi parliament debated its terms, leading members of Congress were obliged to obtain unofficial English translations of texts published by the Arab press.

Bush defends his extraordinary conduct by claiming that it is traditional for commanders in chief to negotiate status of forces agreements without congressional consent. But the Iraqi agreement goes far beyond anything in the traditional SOFAs concluded with close to 100 countries since World War II.

Indeed, it goes far beyond any sensible interpretation of the president’s power as commander in chief. For example, the SOFA creates a joint US-Iraq committee and gives it, not the president, broad control over the use of American combat troops. It thereby asserts the authority to restrict President Obama’s powers as commander in chief throughout most of his first term in office. But under the Constitution, no president can unilaterally limit his successor’s authority over the military.

This defective agreement cannot serve as a valid substitute for the congressional authorization that Bush so casually allowed to expire on December 31. It is up to Congress to authorize continuing military action. Gaining the consent of a foreign power simply isn’t enough.

The question is how Obama should respond to the legal catastrophe that Bush has left as his Iraqi legacy. It’s easy to eliminate one option. Whatever the original infirmities of Bush’s agreement, Obama should not repudiate it. Now that Maliki has won approval from his parliament, the agreement has become the basis for the next phase of Iraqi politics. It also contains withdrawal timetables that are compatible with Obama’s goals: all combat troops out of Iraq’s cities by July; all troops out of Iraq by the end of 2011. As a consequence, Obama may be tempted to accept the agreement that Bush has left behind, and proceed without correcting its obvious constitutional deficiencies.

But this would be a tragic mistake. We are living in an age of small wars—some are blunders, but some will be necessary. The challenge is to sustain their democratic legitimacy by keeping them under congressional control. If Obama goes along with the Bush agreement, he will make this impossible. Future presidents will cite the Iraqi accord as a precedent whenever they choose to convert Congress’ authorization of a limited war into an open-ended conflict.

There is a better way ahead. President Obama should submit the Bush-Maliki agreement to Congress on January 20 and urge its speedy approval. This request is likely to win broad bipartisan support. Rapid congressional ratification will not only fill the legal vacuum threatening the constitutional integrity of our military operations in Iraq. Together with the closing of Guantanamo, it will show that Obama is serious about reining in the worst presidentialist abuses of the Bush years.

Members of the incoming administration have already taken steps in the right direction. Both Secretary of State-designate Hillary Clinton and Vice President-elect Joseph Biden took the lead as senators in protesting Bush’s unilateralism in the conduct of the Iraqi negotiations. And Obama has made clear that he appreciates the role of checks and balances in our constitutional scheme. Now is the time to reverse the precipitous slide toward the imperial presidency.

Bruce Ackerman and Oona Hathaway are professors of law at Yale and the University of California Berkeley, respectively.