How the hell could one ever, forget?
In time, people do forget and become more complacent, and some even try to rewrite history by not talking about it, and even pretending that perhaps it didnt really happen the way it did.
But the shock of 3,000 innocent people on that day losing their lives, regardless of who they were, and what they beleived in, and the utter lack of security in the minds of people changed the way the US thinks... at least until they become complacent again.