AFTER SECRETARY OF STATE Colin L. Powell's presentation to the United Nations Security Council yesterday, it is hard to imagine how anyone could doubt that Iraq possesses weapons of mass destruction. Mr. Powell left no room to argue seriously that Iraq has accepted the Security Council's offer of a "final opportunity" to disarm. And he offered a powerful new case that Saddam Hussein's regime is cooperating with a branch of the al Qaeda organization that is trying to acquire chemical weapons and stage attacks in Europe. Mr. Powell's evidence, including satellite photographs, audio recordings and reports from detainees and other informants, was overwhelming. Sen. Joseph R. Biden Jr., the senior Democrat on the Foreign Relations Committee, called it "powerful and irrefutable." Revealing those tapes and photographs had a cost, as Iraq will surely take countermeasures. But the decision to make so much evidence public will prove invaluable if it sways public opinion here and abroad. At a minimum, it will stand as a worthy last effort to engage the United Nations in facing a threat that the United States could, if necessary, address alone or with an ad-hoc coalition.

Whether Iraq is disarmed through the authority of the United Nations or whether the United States effectively assumes responsibility depends on how the Security Council responds. Though much of Mr. Powell's report was new to many Americans and Europeans, it probably did not surprise the governments that have most strongly opposed action in Iraq, including France and Germany. Diplomats from these nations do not dispute Defense Secretary Donald H. Rumsfeld's assertion that "any country on the face of the Earth with an active intelligence program knows that Iraq has weapons of mass destruction." All supported Security Council Resolution 1441, which said a false statement by Iraq about its weapons, coupled with failure "at any time" to "cooperate fully" in disarmament, would be a "material breach" leading to "serious consequences." None say Iraq has complied. Until now, however, they have cynically argued that the inspectors must uncover evidence proving what they already know, or that it's too early to judge Saddam Hussein's cooperation. Mr. Powell's presentation stripped all credibility from that dodge.

France was ready with a fallback position yesterday. Foreign Minister Dominique de Villepin acknowledged Iraq's defiance of the Security Council and the consequent failure of inspections and then argued that the world should respond by . . . dispatching more inspectors. This hardly qualifies as the "serious consequences" Paris formally endorsed on Nov. 8, but Mr. de Villepin argued, in effect, that a climb down is preferable to war. Indeed, war must always be a last resort, but the French solution offers no credible path to Iraqi disarmament. Twelve years of experience have demonstrated that it is impossible to strip an unwilling totalitarian government of its weapons by such means. As Mr. Powell asked, how could inspections ever determine which 18 of Iraq's tens of thousands of trucks carry mobile biological weapons labs? By choosing such a course, the Security Council would send Saddam Hussein the message that it remains the ineffectual body that shrank from enforcing 16 previous resolutions. By proposing it, France and those who support it are setting the stage for another momentous development they claim to oppose: the transfer of responsibility for countering the most serious threats to international security from multilateral institutions to the world's sole superpower.