Den Beste nails the point the multilateralists would like to avoid:
We will now observe one of those marvelous paradoxes which keep appearing in politics. Since Bush won’t require UN authorization for war, he’ll get it. If the bill which passed Congress had included a requirement for UN authorization, it would not have happened. Isn’t political logic grand?
Now, apply the same tortured political reasoning (a redundant phrase) to the “peace” movement, and you’ll understand why they make war more likely rather than less.