While I'm not going to sit here and exclaim that 9-11 changed everything, I will point out that it did make a lot of people who were reasonably secure suddenly feel helpless. There was a lot of importance to the idea that we DO SOMETHING even if what we did didn't bear any particular relationship to the causes of our discomfort. Whenever the subject comes up, I always like to remind people what happened to the Dixie Chicks. The reaction to Natalie Maines's comment can be described many ways but rational is not among them.
We were led into Iraq because at the time we were ready to be led anywhere and Bushco saw an opportunity in our confusion. To this day, they're still trying to exploit the confusion (see AQ vs AQI) but we're slowly waking up and people aren't buying it anymore.