Generally speaking in international law, invading another country without explicit UN approval is....
I followed the events leading up to the invasion very closely. After the invasion, it became obvious, that no one I knew followed the events at all.
As I remember, after 9-11, the mindset of the Americans changed. But the mindset of Saddam and his supporters did not. But that is not immediately why we invaded. There were countless UN violations that were passed that were designed to inspect and verify Saddams Iraq. Saddam played cat-and-mouse with the inspectors. Letting them in and then shortly kicking him out.
Powel had a point when he said that if some sort of action was not done, the UN would be a pointless body. Their resolutions would not mean anything if Saddam constantly violated them and nothing was done.
And there was a long list of violations.
You can say that it was illegal because the UN did not approve of the invasion in the end. But prior to that, they did pass virtually countless resolutions that Saddam violated. So one can also look at it another way and say that it was good for the UN that Bush gathered "countries of the willing" who joined the US in the invasion.
Let me ask you this. There were many resolutions that demanded that Saddam allow for inspectors that Saddam violated. Now, after the US invaded, did the UN pass any resolutions saying that the US and other countries were wrong to do the invasion?