Depictions in Science Fiction
- The War of the Worlds
- Independence Day
- The X-Files
Considering the sheer scale of our galaxy, let alone the universe, it is quite likely that, like on Earth, life started on extra-Solar planets, and some of that life might evolve cognitive capabilities rivaling our own. A civilization from an intelligent species older than ours could conceivably develop a method of interstellar travel (such as putting the crew of a ship into suspended animation and having it fly through space at a relativistic speed), allowing them to spread themselves across the universe. If they came this way, they could easily destroy our civilization via orbital bombardment and set up their civilization over the remains of ours or subjugate ours with the threat of orbital annihilation, gradually displacing our own. In most conceivable scenarios, humanity would be unable to resist an assault by an alien conquest fleet.
Other depictions of Alien Invasion
Some science fiction portrays alien invasions as a more gradual infilteration/assimalation of Earth and its people over a span of decades to millions of years, perhaps playing off of alien abduction accounts, UFO sightings, and Government conspiracy theorists. A prominent example of this is the 90's television series The X-Files, which depicted an increasingly convaluted alien plan to infilterate Earth governments and create alien-human hybrids. One possible rational for aliens using such a strategy would be if they lacked FTL technology, and so could not deploy the forces for a full-scale attack. However, the aforementioned ease with which even a small force could cripple humanity by orbital bombardment, as well as a time scale which makes terraforming a dead world seem potentially more efficient, such a method of invasion seems somewhat questionable.