There are several good posts at Opinio Juris about the “legality” of the Iraqi war. For me the issue is rather simple: our constitution has an enumerated power that allows Congress to declare war for whatever reason they choose, and no treaty would change that (it would amount to amending the constitution via treaty).
One point they touch on is the legality of the war in Kosovo. This strikes me as a good point to raise: the Kosovo war was popular among many people that oppose the current war in Iraq and they justify it through a fig leaf of multilateralism via Nato. This seems very dishonest to me, since we’ve heard all of the garbage from the transnationalists that the UN is the only body that can “authorize” war, though they never authorized the actions in Kosovo.