(This is an expansion and elaboration on the comments of the ID4 post)
America, the nation that has been the world's leading superpower for most of living memory, the nation that has not had ground warfare on its territory since the 19th century, and, the nation that compared to Europe, suffered very little during the World Wars of the 20th century, continues to fantasize about its own destruction like no other on Earth. Countless films like Independence Day, Transformers, War of the Worlds, etc. revel in orgies of destruction at the hands of singly evil-minded invaders.
These fantasies aren't recent. They see their origins in a subgenre of fiction, "future war", that developed in the late 19th century. These frequently racist stories taking took place usually 20 or so years into in the future, featured hordes of foreigners seeking to conquer the United States and Europe only to be driven back by white superiority and heroism. These stories frequently featured extensive descriptions of the catastrophic destruction of major landmarks of the time, most commonly the Brooklyn Bridge. (This is covered far more extensively in Howard Bruce Franklin's War Stars if you're interested.) Though thankfully stripped of their racist elements, modern-day incarnations of these nationalist apocalypse narratives retain a common essence: invading force cripples the world (ie the US) but is defeated in the end by the pluck, grit, and determination of a few valiant Americans.
The most famous of these future-war stories, War of the Worlds, is the one that most thoroughly subverts the genre. But wait, you say, I saw that movie, isn't it about Tom Cruise learning how to be a good father? Yes, the movie is about that, but the original story isn't. The original story has never been faithfully adapted. It is a bleak tale of humanity's humiliation and subjugation at the hands of merciless, faceless invaders that ends with humanity winning by a fluke. The rah-rah heroism of Spielberg's adaptation is one of the many flaws of that film. But even the role-reversal imperialism of War of the Worlds fails to address the ultimate problem with these stories, with far more problematic implications than nationalism.
When the enemies are aliens or otherworldly creatures, it is easy to overlook the fact that these stories tend to end with genocide. It is a relief to see that our future enemies in today's incarnations of these stories are no longer Chinese, Africans, and other ethnic minorities. The 19th-century future war are a terrifying display of the ubiquity of the racism of the era. In the same way the aliens/invaders are subjugated or even out-and-out exterminated (WotW, ID4) in the modern versions of these stories, the original versions frequently advocate actual genocide against non-white ethnicities. The most famous of these would have to be "The Unparalleled Invasion" by Jack London (yes, the Jack London) where, in the climax of the story, the entire Asian race is exterminated via bacteriological warfare. This depicted as a good thing.
To my knowledge, only one story, Ender's Game for all its flaws, addresses the inherent genocide of these stories, but not explicitly. In Ender's Game the genocide is the ultimate expression of the novel's underlying philosophy, that morality is dependent entirely upon intention (more on that argument here).
We conceive of ourselves as good and inviolably sacred. We must eradicate evil, or those that would threaten us. Even someone as revered as Saint Obama holds this opinion: during the presidential debates he said, verbatim, "We will crush Al-Qaeda and kill bin Laden." Crush and kill. Opposing ways of life cannot coexist. This is why the natives of the continent were so brutally driven from their land. Their way of life could not be made compatible with ours. We were the ones with the numbers and the guns, so we prevailed.
This is why September 11 shook us so badly, the catastrophic destruction of a national landmark exactly in the manner of our fantasies. "It's like something out of a movie," any number of bystanders were heard to have said that day. This is why a single terrorist attack could yield so much political leverage.
The stories of future wars are the secret fear we have that someday we won't have the numbers and guns. That someday the might of another nation or people (even if they are alien) will prove our way of life will be the wrong one, and that we may no longer continue existing. We always win in the end in these stories, and that is why we love them. They validate us.