Under the terms which ended the last Gulf War, Iraq was required to do a lot of things that it has failed to do. This failure constitutes grounds for us going to war based on the previous agreements. In effect, it means that the Gulf War never ended.
On the other hand, if we were to want to go to war against Iran, for example, we would have to prove conclusively that they were directly involved in an attack on this country, as was manifestly true in the case of Afghanistan. Even then, after all this time, it would be an uphill fight.
But no such proof is necessary against Iraq. We can go into Iraq just because they have not kept the terms they signed ten years ago.
To which I say, "Works for me!" In order to combat Islamic terrorism, we are going to have root out these evil regimes wherever we find them. Iraq is as good a place to start as any.
We all know that there are many places where terrorists can find support: namely almost any place where there are Islamics and enough chaos or local autonomy to protect the terrorists. This includes parts of Pakistan, Indonesia, the drug cartel areas in South America, numerous countries in South Asia like Chechnia, etc. "Evil regimes" is a convenient substitute for the much messier and harder problem of evil areas.
The best way to fight terrorism is to directly confront terrorists wherever they are. But finding them is an even harder problem and the chaos that would result from a civil war in Iraq and neighboring areas would make that job harder.