Steven den Beste is spot on in this analysis of why we were attacked. It was nothing to do with American foreign policy - it was American ideas.
We have no problem living side by side with them, but to Islamic extremists we are a stark danger – and we would remain a stark danger even if we militarily disengaged from the world, stopped supporting Israel, and made all the other concessions that some suggest we should. Because it isn't the Third Armored division which they fear; it's television and radio and fashion and the Internet. It's bikinis and Saturday night dates; it's rock-and-roll. It's comfortable clothing. It's Saturday in the park, and hanging out at the mall after school. And it's our women, our damnably independent women, who not only demand equality with men but have proved that they deserve it by performing just as well as men. They fear our women, because they fear their own women.
In actuality, they attacked us out of self defense, as they viewed it. They were attempting to defend their faith against the heretical influence of our culture, and the slow but sure way that it is destroying what they see as the true practice of Islam. And as long as we believe in things like freedom of expression, and freedom of behavior, then to a greater or lesser extent we will continue to eat away at the roots of Islamic culture simply by existing.