Mr. Bush's choice to fully confront this challenge has been precipitated by two developments since his election. First came the crumbling of the containment policy that Mr. Clinton relied on to manage the Iraqi threat; then came 9/11. The administration's attempts to explain the implications of these events have been awkward and sometimes confused. It has asserted that Saddam Hussein has connections to the al Qaeda network but has provided no public evidence that this is so. It also has suggested that terrorists could strike the United States with chemical or biological arms supplied by Saddam Hussein; though this is plausible, again there is no evidence that the dictator has adopted such a strategy. The real case for acting now on Iraq is more intangible: It is that the breakdown of containment, and the new flow of resources that breakdown has provided to Saddam Hussein, has decisively raised the cost of postponing a confrontation; and the shock of 9/11 has given this country the lesson that, in an era in which enormous harm can be done by seemingly weak adversaries, threats such as that posed by Iraq must not just be managed but treated aggressively.

With his speech to the United Nations and his request for a resolution from Congress, Mr. Bush has set his administration on a rapid march toward a decision on war. Though he has nominally reserved final judgment and left notional space for consultation, the president has practically excluded any outcome other than a U.S.-led military campaign that, beginning in just a few months, would seek to destroy Saddam Hussein's regime, locate and neutralize its weapons, and replace it with a progressive and pro-Western government. The urgent pace of the political and diplomatic offensive, which demands congressional and Security Council action within weeks, and its timing at a moment when the separate battle against al Qaeda is far from won, gives us pause. Nevertheless, we believe that the president's decision to act is the right one, as is his challenge to the Security Council to support the enforcement of its resolutions. Though the timing and wording of a resolution are open to discussion, Mr. Bush deserves Congress's support.

Barring the unexpected, action will mean sending tens of thousands of young Americans to fight a potentially horrific battle against a hardened war criminal who can be expected to use every weapon left in his arsenal. There can be little doubt that the fight will be won, perhaps even quickly. But as it faces this grave prospect, the administration must do everything in its power to minimize the war's risks and ensure that the promised outcome -- a disarmed and united Iraq, at peace with its neighbors and the United States -- is realized. So far, there are worrisome signs that this has not been done. In particular the administration has failed to show that it is prepared to manage the daunting task of occupying and managing a post-Hussein Iraq.

Secretary of State Colin L. Powell and Secretary of Defense Donald H. Rumsfeld each delivered lengthy statements to Congress last week arguing the case for action; strikingly, neither so much as mentioned the reconstruction problem. When questioned by members of Congress, the secretaries offered sweeping assurances: that postwar Iraq would have a representative government that would help transform the region around it; that the burden of occupation and the cost of reconstruction would be shared by the United Nations and numerous allies; and that U.S. occupation troops would not be needed for a prolonged period. Yet there was no hint that a coherent postwar plan exists; Mr. Rumsfeld suggested it would be left to Iraqis to work one out after the war.

The Bush administration has used similarly expansive rhetoric about its commitment to postwar Afghanistan; the reality has been a pinched and narrow-minded effort that has left the country dangerously close to lapsing into anarchy. The same mistake cannot be made in Iraq: An intensive and prolonged project of nation-building -- similar to that undertaken in Bosnia or Kosovo -- will be needed. This, as Mr. Rumsfeld acknowledged, will require major contributions from the United Nations and allies. And these will not be easily obtained unless the administration continues to pursue a policy of coalition-building, both in and outside the Security Council -- a policy that may require expending more than the few weeks the administration seems to have allotted for it and allowing a last U.N. attempt at a peaceful solution. Mr. Bush is right to insist that the civilized world at last end the menace of Saddam Hussein, and threats of unilateral U.S. action may help bring that about. But a military campaign undertaken in haste, without substantial support from other nations or before careful planning about Iraq's future, would incur great risks. Avoiding them should be the focus as Congress works with the administration toward a resolution of support.