The war (or, if you prefer, the determination that the conditions for the cease fire from the original Gulf War had been violated) had zero moral or legal justification. There's a lot of post-hoc rationalization going around that the war was based on false intelligence. Had any of those widely believed intelligence reports that are now known to be false had in fact been true, there still would have been zero moral or legal justification to attack Iraq, which had not attacked us and was not even capable of an imminent attack.
With regard to the intelligence for the war, the United States set a standard of proof for the claim of an imminent threat way back in the early 1960's, during the Cuban Missile Crisis. Our government didn't even meet that standard (convincing photographs) in 2003, despite the advances in technology that had occurred from late 1962 to early 2003. From what I could see, there wasn't a credible argument that Iraq posed an imminent threat.
If there had been a misperceived imminent threat, then as far as I can tell, it hadn't grown in magnitude from the time that Congress gave permission to pursue the military option, which in my mind means that anybody who disagrees with me about whether an imminent threat existed should fault the Executive Branch of the American Government for taking so long to respond to whatever threat they supposedly were responding to. (In other words, if the President thinks that the Easter Bunny and his herd of flying unicorns are getting ready to tow their Winnebago of Death across the Atlantic Ocean in order to gas Americans, then the President needs to address that threat the moment that she perceives it, not after months and months of wasting everybody's time pretending that the U.S. needs to get permission from the United Nations and then deciding that, on second thought, the U.S. doesn't actually need that permission.) In fairness to the Executive Branch, the Congress asked them to pursue diplomatic options first. Still, if you don't actually feel like you need to get a particular endorsement for an action, don't waste time pretending you do. All that does is give the Easter Bunny more time to sharpen the unicorns' horns.