People often consider the 2000 X-Men film to kick off the explosion of superhero films in the new millennium, and that was pre-September 11, so I’m not sure that correlation is causation here. Moreover, a lot of European artists in the early–mid 1960s remarked how superheroes were oddly prominent in American pop culture, and at that point the USA was still on top of the world and hadn’t been shamed by Vietnam.