(crap, i cut and pasted the same argument twice)
A United-Nations-League-of-Nations-type thing is good as a forum and as a way to resolve some international crises, but it absolutely doesn't work in some situations. RUSSIA is sitting on the Security Council, with a veto; why the heck would they EVER support an action that destroys their economic interests, regardless of how "just" it is? Veto, veto, veto, and how interesting that the opposition and support in the UN went completely along profitable lines? (Germany, France, Russia, and Belgium all have something big to gain if the war doesn't happen; the US and its allies have something big to gain if the war DOES happen; caught in the middle...is Iraq)
Even if you don't think this particular war is "just", how can you advocate creating a situation where you have to say to the citizens of a country: "Sorry about the crimes against humanity being committed, but the guy committing them was smart enough to get into business with someone who has a veto. We can't do anything."?