Legality of Iraq war
No war is legal or illegal, there's only moral or immoral -- and all wars are immoral (some just more than others), the only real issue is whether the war is more immoral than doing nothing. Now as for legal justification, there was more justification for the Iraq War than most of the wars we got into.
Legal is not always moral, and vise versa
That still doesn't make it right, it just makes it slightly less wrong. But we had support from U.N. (1441), NATO, largest coalition ever, and a foreign country that had broken the terms of the cease fire. That's more than we had for any other war, included Korea (which was the previous war with the most faux-legality behind it).
The facts are the U.N. was a creation of the U.S. to share power and give others a say. Using their approval or denial has little to do with legality, and a lot more to do with whether you have consensus for your immorality.
More than just U.N.
There are many alternatives and many forms of law. NATO makes rules and laws. Treaties are laws and rules. And if you build a coalition of 40 nations, then you have some law supporting your actions. You might not have enough law for some people, but we certainly had some legal standing for the war by the coalition itself.
Before the war we passed multiple resolution in the U.N. The most significant was Resolution 1441. Our congress passed a law authorizing the invasion of Iraq a month before 1441 was passed and the entire U.N. and everyone knew exactly what that was about. The French wanted a soft resolution that said, “be nice Saddam or, or nothing will happen”. We fought for the far more rigid 1441 which said that Saddam had to prove that he had no more WMD’s (after he’d used them) by a cer- tain date, and that he had to let the U.N. inspectors go anywhere they wanted; OR ELSE. The “or else” was a huge contention because the French and U.N. knew that meant regime change by force. We’d stopped short in the first Gulf War because of the U.N. and Arab allies had wanted it; but that had cost hundreds of thousands of lives and the suffering of millions for a decade. We’d given the U.N. enough time to fix things, and we were done waiting, and the world knew it. We fought over the words in the resolution because they knew what it meant, and we won. Playing innocent after the fact, or claiming that we needed more approval or this was unilateral is 100% wrong. It would have been nice to get more, but not necessary.
Saddam was the cause
Saddam failed to live up to the resolution, and it wasn’t even close. He broke the law multiple times. He shot at our planes in no-fly zones and kept lighting them up with hostile radar, these are known as acts of war. He paid terrorists to kill our allies. He aided and abetted enemies and gave them cover in his nation. He had corrupted the U.N. with bribes, built secret bunkers, facilities and weapons systems. Only after we spent billions putting a large force on his border, did he agreed to give some more limited inspections, but never followed through on the letter of the law (proving/ accounting for the WMD’s he had). So we had enough, and had enough of an excuse.
The U.N. (Hans Blix) said in January that Saddam had not complied (or with the dozens of resolutions before that). Hans started getting softer in March when political pressures came down, and they realized we were serious. Then everyone started making excuses; but it was obvious they had not met the terms of the resolution, as he had not for the prior decade. (Waiting 10 years for the U.N. to fix the problem is not exactly a rush to war).
U.N. resolution 1441 was more law (more legal) than anything we had in Kosovo (or later Libya) or for most other wars . Still, we went back and tried to work with the U.N. even more. 40 Countries and a resolution made it obviously not a unilateral decision, but there could never be too much support. But alas, many stonewalled and blocked us. We still would have gotten another resolution passed anyways, and everyone knew it, but France vetoed us (that’s why they vetoed because otherwise it would have passed, remember?). So we acted on the authority we had already.
NOTE: I argued against the War before we went in. I didn't like the lack of an exit plan. Never get into a fight, until you know how you will get out of one. But even though I wasn't a supporter of going to War (yet), that doesn't mean I'll deny the obvious, and pretend that we didn't have legal justification to do so.
Some people see what they want. They see it as unilateral to work with the U.N. for dozens of years, to found the U.N., to pay the hogs share for the U.N., to fight the wars for the U.N., to do the same for NATO, to get a resolution, and to build a coalition. But the truth is that we had more than enough legal justification; we had more UN approval for Iraq than we got for Bosnia / Balkans2, we had far more than the French ever got for Angola, than the Chinese ever got for Tibet, than the Russians ever got for any of their actions, we had more than the UK got for the Argentina/Falklands. And no country needs approval from others, in the same way you don't need approval to defend yourself from a mugger. You might be punished for self-defense -- but that doesn't mean you made the wrong decision, because it's still better to be tried by 12 than carried by 6. Every Country in the world, will make decisions based on what they believe their interests are. America is one of the few that considers the interests of others. (Far more than any of our biggest critics).
So before anyone says that we didn’t have legal standing, they should probably think about how ignorant they will look. Then take a more mature/moderate approach. Arguing that even though the U.S. did more than anyone else does or has done, and they had more legal standing than any other conflict in recent history, some might still think we should have done more, or not gotten into the fight -- and that's fine. Just don't be stupid and argue the War was illegal. We lived up to a higher standard than anyone else in history; so don’t try to rewrite history.