Fred Kaplan in Slate reminds us that this isn't even going to be a preemptive war:

...the war that Bush II is pushing is a different sort of war, a war in which we launch an invasion, not in response to aggression and not even "pre-emptively" (to strike the first blow before the other country does) but "preventively" (to keep the other country from doing something that might let it pose an imminent threat someday).

Then he makes what should be a thuddingly obvious point:

There may be a case for preventive war, but if the aim of the war is protecting the international order, then that case should be acceptable to the agency that represents the international order. Specifically, if the war is supposed to enforce a U.N. resolution, then the case for war should be acceptable to the United Nations.

A couple of paragraphs later, he makes another one:

If the administration lacks the acumen or persuasive power to deal with such familiar institutions as the U.N. Security Council or the established governments of France, Germany, Turkey, Russia, China—even Canada—then how is it going to handle Iraq's feuding opposition groups, Kurdish separatists, and myriad ethno-religious factions, to say nothing of the turbulence throughout the region?