WITH U.S. FORCES camped in the center of Baghdad last night, it appeared that Saddam Hussein's tyranny over Iraq had been all but broken. Yet the United States and its allies still lack a workable plan for what will replace it. As President Bush traveled to Northern Ireland to discuss postwar strategy with British Prime Minister Tony Blair, U.S. commanders were preparing to launch a temporary administration made up mostly of U.S. officials and diplomats, with the announced aim of maintaining basic services and delivering humanitarian aid. The allies agree that at some point much of that administration will be turned over to Iraqis, and that eventually an interim Iraqi authority will be created. But there is no consensus on what will likely be the central postwar problem: how to conduct a transition that is accepted as legitimate by most Iraqis and makes possible a sharing of the burden of reconstruction among the United States and other nations.

Mr. Blair and leaders of Congress from both parties have been urging the Bush administration to give the United Nations a central role in the process, and for good reason. Though the U.N. Security Council failed to follow through on its resolutions ordering Iraqi disarmament, the institution has played an important postwar role in every conflict the United States has entered since the Cold War. As Secretary General Kofi Annan indicated yesterday, a comprehensive U.N. administration in Iraq is not appropriate or necessary; but U.N. participation in, and sanction for, the process by which Iraqis form an interim administration will give that fledgling regime legitimacy it otherwise could not have. It will also open the way for European, Arab and other governments to help with reconstruction and security, which in turn will allow U.S. and British combat troops to withdraw more quickly. Finally, U.N. postwar involvement could help mend the breach in the Security Council and prevent the United States from being perceived in the Middle East as an isolated occupying power.

It may be that France and others who lined up against the United States before the war would again obstruct a Security Council consensus, thereby making U.N. participation impossible. But at the moment the main obstacle to a multilateral process appears to be the Bush administration -- and in particular the Pentagon's drive to dominate every aspect of postwar Iraq. Administration officials argue that having borne the burden of the war, the allies have a right to that dominance; but Mr. Blair clearly does not want the privilege, and U.S. officials have yet to persuasively explain why it would be in the American interest. True, unilateralism might allow the Bush administration to control which Iraqis emerged as provisional leaders -- but the Iraqi leaders it chose exclusively would be at risk of being regarded as puppets, whereas in postwar Afghanistan the administration managed to exercise the same influence under U.N. auspices. U.S. dominance could make some postwar reconstruction programs quicker or more efficient -- but it would also mean that the United States would have to bear far more of their costs. Tiresome negotiations with France and Russia might be avoided -- but at the cost of perpetuating a transatlantic divide that threatens to destroy NATO.

Mr. Bush pledged before the war to seek "partnership" with the United Nations in Iraq, and officials were saying yesterday that the president's differences with Mr. Blair were not really that great. We hope that's the case. But much of the rhetoric coming from the administration suggests the White House hopes to compel Mr. Blair, and after him the Security Council, to go along with the unilateral scheme that the Pentagon is already poised to implement. Such a plan will maximize the costs and risks to this country -- as well as the chances that the process will fail. For the sake of the international system and the Western alliance, but even more for the security of the United States and its troops, Mr. Bush should work with Mr. Blair to create the multinational partnership for Iraq that he promised.