A legal time bomb in IraqObama must return US foreign policy to the rule of law - and the mandate for war is about to expire

Hillary Clinton's first task as US secretary of state will be to defuse the legal time-bomb that the Bush administration has set up in Iraq. Up to now, the military occupation has been authorised annually by the UN. But now the administration plans to let the UN mandate expire on December 31, and replace it with a new "status of forces agreement" recently approved by the Iraqi parliament.

But the Bush-Maliki agreement only covers American forces. Once the UN mandate expires, there is no longer a legal foundation for troops deployed by Britain and the other remaining allies. While Britain is planning to leave Iraq next year, it is seeking its own bilateral agreement for the interim. But time is running short. Most of the other allies are rushing for the exits, and with good reason. Any soldiers that remain on January 1 will be in the country illegally and will have no protection against prosecution in Iraqi courts. The "coalition of the willing" is coming to an ignoble end.

To top it off, the termination of the UN mandate will leave American troops without authority under US law to engage in ongoing combat. In granting President Bush the authority to invade Iraq in 2002, Congress limited it to two purposes: to "defend the national security of the US from the threat posed by Iraq" and to "enforce all relevant UN security council resolutions".

The government of Iraq is no longer a threat to US national security, so the first clause no longer applies. Indeed, the Bush-Maliki agreement proclaims that "the danger posed to international peace and stability by the former Iraqi government is gone now". And once the UN authorisation expires, there will no longer be a relevant security council resolution to enforce. Since neither clause applies, the use of combat troops will become illegal on January 1.

The bilateral agreement with Iraq does not fill this legal hole. Bush has concluded the agreement on his own without giving Congress a chance to vote on it. The agreement is his and his alone. But the president does not have the power to wage war on his own.

Constitutionally, he needs Congress approval - which is precisely why he sought authorisation in 2002. When challenged on this matter at a congressional hearing, the administration representative, David Satterfield, was initially unable to give an answer. In a later written response, he cited Congress's 2002 authorisation - the very one that expires at the end of this year.

Satterfield also cited the resolution passed by Congress immediately after September 11, authorising the president to use force against the terrorists involved in these attacks. But the resolution was intended to endorse military action in Afghanistan, not Iraq. This was why the president had to return to Congress the next year to obtain explicit authorisation for the Iraqi invasion.

In a last-ditch effort, Satterfield pointed to Congress's decision to vote a supplemental appropriation for the war until June or July of next year. But the supreme court has held that appropriations are not the same thing as direct congressional enactments. If they were, a limited authorisation for military action, like the one Congress passed in 2002, would always become an open-ended commitment to unlimited war.

All this is typical of the Bush administration, whose cavalier treatment of the rule of law has embarrassed America. Its extreme unilateralism leaves the incoming administration in a difficult position. Technically speaking, it only comes into power on January 20. But Hillary Clinton shouldn't wait. She should immediately take steps to encourage the Maliki government to extend the terms of its agreement to Britain and any nation that wishes to remain. If she quietly puts the Obama administration behind this initiative, it is far more likely that Maliki will push for parliamentary approval.

Solving the legal problem in America is trickier. As a leading participant in the Senate debates, Clinton understands Satterfield's distortions. In fact, she was one of the first to demand that the Bush-Maliki agreement be submitted to Congress. She should continue pressuring the White House to take this step. If Bush persists to the bitter end, she should urge Obama to submit the Bush-Maliki agreement for congressional approval as soon as he takes office.

President Obama must return American foreign policy to the rule of law. It is time for him and Clinton to demonstrate that the era of illegal presidential unilateralism has come to an end.

• Bruce Ackerman and Oona Hathaway are law professors at Yale and the University of California, Berkeley, respectively bruce.ackerman@yale.edu