Invading Earth for our resources or our women, serving man, or serving him on a platter, enacting a glorious golden age of peace and love or simply enslaving us -- the idea and possible outcomes of an alien invasion is a pervasive and exciting theme in science fiction.

We start with the genre-defining alien invasion novel, H.G. Wells' War of the Worlds proper. Published in 1898, Wells' story is so well known as to be almost a cliche in and of itself: After a series of Martian explosions, strange meteors crash land in London, disgorging cephalopod-like Martians and their massive Tripod war machines. Using a heat ray and a series of chemical and biological weapons, the Martians quickly subjugate Britain, only to be wiped out by common Earthen bacteria.

War of the Worlds wasn't merely the prototypical alien invasion novel. It was both a political parable -- Isaac Asimov believed it to be a denunciation of African colonialization -- and a gasket for political tensions. Invasion novels had become increasingly common in the last decades of the 19th century as mounting political tensions sent Europe gradually lurching on its way to the first great War. Wells' novel was a perfect capsule of parable, moral and escapist adventure.

It is no wonder, then, that alien invasion scifi has resurfaced time and time again, during periods of political uncertainty... but not during periods of actual war. The '30s and 40's feature a relative dearth of alien invasion fiction, but with the '50s (and the beginning of the Cold War), science fiction became invasion mad.

Consider 1951's The Day the Earth Stood Still. It is not technically an alien invasion novel, as the aliens never invade: However, Klaatu is essentially a galactic emissary, threatening Earth with invasion and possible obliteration if it doesn't keep its warmongering planet bound. War of the Worlds was first made into a film in 1953, similarly highlighting Cold War concerns. Alien invasion films were ubiquitous with scifi in the 1950s: Invaders From Mars, Plan 9 from Outer Space, Invasion of the Body Snatchers... the list goes on. Nor did scifi novels show restraint: Famous scifi novelist John Wyndham wrote no less than three alien invasion masterpieces during the decade: Day of the Triffids, The Kraken Wakes and The Midwich Cuckoos. And Robert Heinlein wrote his classics The Puppet Masters and Starship Troopers during this same decade, each very clearly spelling out a Cold War parallel.

Alien invasion as a scifi zeitgeist lost a bit of its luster in the '60s and '70s as real world tensions flared up -- the Cuban Missile Crisis, the Vietnam War -- Americans didn't need to look to the stars for potential invaders. But the '80s proved another fertile decade for alien invasion: NBC's miniseries and subsequent television series V set flying saucers hovering over major American cities, piloted by strange reptilian aliens named "Visitors." Orson Scott Card's novel Ender's Game also focuses on an invasion of insect-like aliens that can only be thwarted by the greatest military genius in human history: A child. And offal though they may be, it's worth noting L. Ron Hubbard's Battlefield Earth series was both written and became popular during the '80s.

But something interesting happened in that decade: The political cycle of alien invasion fiction weakened as video games became popularized. Aliens became easy, popular and culturally universal antagonists. Tecmo's Space Invaders started the trend, channeling the plot of War of the Worlds into a simple black-and-white arcade game. As technology got more advanced, so did the games: id software's Doom is essentially a story of alien invasion, where the "aliens" are eerily similar to what we would consider to be demons. The fantastic mid-'90s strategy game X-Com starred an elite force of commandos defending Earth from invasion by a race of aliens. Valve's incredible Half-Life series is devoted to the invasion of Earth and the subsequent resistance; Bungie's Halo series stars a superhuman cyborg as Earth's last hope against an endless armada of religiously zealous extra-terrestrials. And so on. Alien invasion in video games isn't so much a topic as the brunt of the genre.

Needless to say, though, with political tensions again on the rise, the millennium hasn't been stranger to alien invasion films. This decade alone has seen M. Night Shamlyan take a stab at the sub-genre with Signs and another remake of Invasion of the Body Snatchers. Fox even dusted the X-Files off after a hiatus of nearly a decade. Half-a-dozen alien invasion films are in production. The only other genre of science fiction doing so well is the post-apocalyptic. Clearly, when politics get tense, scifi today steps in and fills the same role that the very first alien invasion novel filled over a century ago: It offers escape from our troubles, it parallels, it predicts and fulfills.