If the war in Iraq was illegal, why was there a UN resolution and mandate?

[Prometheus I'm Not]

I asked this a liberal professor this question, thinking that I had gotten him, and he was rather nonplussed about the whole thing. I think it's another example of liberal "doublethink," where they know truth from fiction, but they need to adapt the belief in one or the other as their handlers dictate.
EDIT: The facts are there. Just google it. It was a legal war and a legal occupation.