There's a lot to read on the subject 9/11. Here's something from Andrew Sullivan, it's very compelling.
Near the start he writes:
We need to understand that 9/11 worked. It worked as a tactic to induce American self-destruction, even if it failed spectacularly as a strategy to advance Al Qaeda—and its heretical message of suicidal warfare—across the globe. It worked because this was not just another terror attack. The emblems were clear: the looming towers of Western capitalism in New York, the cradle of Western democracy in Washington. When the third plane crashed into the Pentagon and the fourth (United 93) was brought down by its passengers, the drama didn’t cease. We saw the symbol of America’s military preeminence lying with its side opened like a tin can.