I was unconvinced Iraq was an imminent threat at the time, and always considered the invasion a violation of the U.N. charter. But these are qualitatively different invasions.
There were around a dozen U.N. resolutions directing Iraq disarm, stemming from their prior invasion of Kuwait. Which, interestingly, Iraq complete their last reparations payment to Kuwait just last week. Saddam Hussein was consistently evasive and uncooperative with the U.N. inspections regimes. None of these are adequate justifications to invade Iraq, in my opinion. But they are some of the legal ones.
Neither the U.S. nor U.N. questioned the borders of Iraq, or the right of Iraqi nationalism. Russia categorically rejects Ukraine's right to exist as an independent country. Keep in mind this invasion started in 2014 with the taking of Crimea. Something the west wrongly accepted, and let Putin get away with it. That was clearly a dress rehearsal for what is happening today. How would things be different if meaningful sanctions were levied on Russia for that? How many lives would be saved? We will be getting some idea.
Also, U.S. attacking Iraq had no chance of starting world war three. Russia invading Ukraine could. At what point does Russia consider western countries arming Ukraine a reason to take a much bigger retaliation, not against Ukraine? And at what point does one of those trigger NATO article 5? The risk here is very different than it is was with Iraq, as wrong and terrible as that was. Putting an end to what's going on in Ukraine is important for everyone, because unlike Iraq, everyone globally has something to lose. World war two took nearly 100 million lives, 1/3 due to famine.
Iraq was never going to be a world war three. And I will never understand the legitimacy of that war.