Legal is a pretty relevant term considering every country has their own opinion of what legal is. When it comes to congress, No I do not believe it was "legal." When people hear of the Iraq War they will usually start blaming Bush and go on some type of "I hate everyone who disagrees with me" rant. Needless to say pointing fingers and crying gets us nowhere! We should most assuredly learn from the past, but we should never dwell on it. Thank you for hearing me out and may God bless you all.
This is not valid to have an opinion about.
If you mean as far as the government was concerned, maybe it was "legal". It would make more sense to ask if the invasion (it's farcical to call it a war which implies a certain degree of mutual action...) was justified, such as whether Saddam's fabled WMD existed.
Opinion of people around the world was overwhelmingly opposed to the invasion - especially in the Middle East and Latin America. So not only was it illegal, it was nearly universally reviled. The USA was feared and condemned for its illegal imperialist war.
And Iraqis today have little left but a massive increase in sectarian conflict. The Iraqi invasion contributed to local tensions so that the Middle East became a largely unstable place. Saddam was holding the reigns of the war hounds 'Sunni' and 'Shi'ia' and now it's not very pretty. The aggressors hold the whole responsibility for the consequences of initiating an illegal unjustified war, and as such the USA and its allies like my country Norway, are responsible for hundreds of thousands of deaths, and too much to mention that should not be needed to recount in detail here, although I must say it is tempting.
The Iraq War was illegal. The war was not justified at all. Sure, Hussain was a horrible leader of his people and great crimes and injustices were taking place in Iraq, but the weapons of mass destruction were in Afghanistan, and that was what the US was primarily looking for.
The Iraq war was Illegal because there was never a formal declaration of war by congress. There is also the fact that the justifcation for the war--Iraq having WMDs-- was a lie. Those that caused the war should be tried for war crimes because of this. In particular, those that knowledgeably lied in order to sell the war for private profiteering and political gain.