Actually on reflection I think Vietnam was part of this broad picture. People saw it as a loss of innocence, and America lost: it was the first crack in the armor. There was soul-searching and the beginnings of self-doubt. But 9/11 was different because it struck at the heart of America, its core symbols and institutions, and innocent civilians were seen jumping to their deaths from a burning building. This unleashed unholy vengeance, and I think that changed America in its own eyes, even if America had in truth always been this self-serving behemoth of military and financial might.