Surely "legit" isn't a question of whether it's a good idea or not, but of whether it is legal. And for it to be legal certain conditions need to be met. In principle a war on any country would be legal if those conditions are met.

So the question is what condition need to be met, and have they yet been met in the case of Iraq. My understanding is that approval bythe Security Council - with nine votes, and no vetos by permanent members - would be needed. And tht hasn't happened so far.