I believe that the real reason the United States invaded Iraq is not something mentioned on a regular basis, the rationale dates before 9-11, and oil has a whole lot to do with it.
The story actually begins in the Summer of 2000. June 10, to be exact. Bill Clinton was still in office, and Mohammed Atta had been in the United States a whole week, after having secured a place for flight lessons. The GOP Convention where George W Bush was first nominated was not to be for another month and a half.
June 10, 2000 was the day Hafez Al-Assad died. Given that both Hafez Assad and Saddam Hussein were part of Ba'athist movements that had ultimately seized power in their respective nations, and had both ruled their nations for a very long time, it would not be unreasonable to watch what happened with Syria as an illustration for what would happen were Saddam Hussein to die while in office.
The succession of Bashar Assad to Syria's "presidency" was, by all accounts a very smooth transition. Thirty years of absolute rule was long enough for Hafez Assad to clear away all threats to his successor. Much was made of his past as an opthamologist who had a liberal education. Then he, to no surprise, turned out to be much like his dad.
Camp David negotiations broke of a month and 15 days later, and the Palestinians were again armed for a fight. On September 27-28, the fighting started again. Considering the number of Palestinian leaders hiding in Syria, can one imagine finding a new source of funding may have pulled Arafat from the negotiation table. Guess who was suddenly finding themselves freer to act?
Now, applying the lessons of Syria to Iraq, it could be surmised that without intervention, upon Saddam's death, one of his sons would rise to power, and very likely start doing those things Papa couldn't get away with. On top of that, Iraq was an OPEC nation, and was actively bribing various nations with oil rights using Oil-For-Food. So it looked like sanctions on Iraq would likely die with Saddam. On September 10, 2001 this was a moderate threat, and kind of an annoyance, but what could you do?
In the wake of the next day's events, and the subsequent War On Terror, the question of Iraqi succession was no longer annoying and inconvenient. We could no longer afford to lose sanctions on Iraq and wait for Uday or Qusay to fornicate upwards enough to re-merit the sanctions. Given all the bribes given out by Saddam, that would be a fatally dangerous wait.
And so, we invaded.