Alien invasion is a science fiction theme in which extraterrestrial life attacks Earth with the intent to conquer it. The classic treatment was The War of the Worlds by H.G. Wells.

Other treatments have posited biological invasions (Invasion of the Body Snatchers), or cultural invasion (The Uplift Wars by David Brin).

Alien invasion was a common metaphor in science fiction during the Cold War for the fears of the American people.

Why invade?

As any scientist will say, Earth appears to be one of the few planets in existence which possesses large supplies of liquid water, mineral resources, a temperate climate, a protective and varied atmosphere, and a large number of vegetable and animal species living together in symbiotic ecosystems. It is widely agreed that these are the necessary prerequisites for a species to evolve beyond subsistence, animal level, and become an intelligent lifeform capable of developing technology. The home planet of the alien species would most likely be very similar to Earth, with similar environmental issues. If problems on the alien planet were beyond the dominant species' capabilities, it is conceivable that they may migrate to another similar planet.

There are various reasons why an alien planet may be rendered uninhabitable. Overpopulation, ecological disasters, the after-effects of war, or possibly factors beyond the aliens' control, such as the collapse of their nearby star or the aftershocks of a hypernova. It can be argued that if the aliens' homeworld was rendered uninhabitable, and they possessed space-travel technology, they may seek to move to another planet. Expansion could also be due to economic and/or political reasons, such as colonialism and imperialism.

However, these factors are highly unlikely to prompt a species to leave their own planet. The effects of ecological disasters or war on the alien planet would be repairable, and would not necessitate abandoning their homeworld, with which they would have a strong psychological link. Colonialism would not pose a significant threat - Earth lies at the very edge of the Milky Way galaxy, and does not (from our viewpoint) have any strategic value. An alien civilization would also doubtless be aware that the dominant, industrialized, species on Earth had already inflicted considerable ecological damage upon their own planet, and would likely possess the capabilities to destroy much of the planet's surface using atomic technology in the event of an invasion by a species immune to conventional human weapons. The logic behind the reasons for attacking Earth is seriously flawed - whilst Earth has rich resources, the current dominant species has already caused significant damage and has the power to cause much more, whilst the reasons for the aliens leaving their homeworld are equally unreasonable. However, if aliens did have the technological capabilities, political and economic motives, and ideological desire, would they invade another planet?

Would aliens invade?

It can be persuasively argued that an alien civilization would have no desire to conquer Earth. If one looks back through human history, countless examples of invasion, conquest, slavery, and genocide, with their resulting trauma and devastation, can be found. It is almost certain that an alien civilization will have been through similarly horrific experiences on their own planet, and possibly during encounters with other alien life-forms. Therefore, an enlightened and technologically advanced civilization would be eager to avoid repeating their own history, and would seek peace and co-operation with another intelligent species. Therefore, if aliens were to arrive on Earth, they would almost certainly "come in peace", on a quest for scientific and cultural knowledge beyond the confines of their own species. This, of course, is based on the assumption that the alien society possessed the ability to travel immense distances through space, and was aware of Earth's existence.

However, it can equally be argued that an alien civilization would be aggressive and hostile. After all, if a species had acquired the technological capabilities to travel through space, military applications must have played a key role. Most of the everyday technologies that we humans enjoy in the 21st century are simply variations of earlier military technologies developed during the 20th century. Besides, the alien civilization could easily be oriented towards aggression and a military lifestyle - there have been numerous examples of this in human societies. If this were the case, we can safely assume that the military capabilities of the alien invaders would be unimaginably superior to human weapons systems.

Earth Defense

An alien civilization capable of finding Earth in the vastness of space must obviously possess unimaginably superior technology and strategies than the human race, which is still in a technological infancy. If, as many science fiction works imagine, Earth were to be attacked by a fleet of alien spaceships, we would have no defense. Astronomers could conceivably detect an approaching space fleet some time before its arrival - assuming of course that the fleet was traveling slower than the speed of light (see theory of relativity). A fleet of alien warships would clearly possess very sophisticated weaponry, and the few weapons that Earth could use in space - nuclear-tipped ICBM rockets - would doubtless be completely ineffective. A land invasion by alien forces would be unnecessary, as most of the human population could undoubtedly be eliminated from orbit, but in the event that a ground army was deployed, humankind would be rapidly defeated. Earth's conventional weapons systems would almost certainly be useless against extraterrestrial technology, and an invasion would most likely result in the human nations deploying nuclear weapons in a hopeless effort to defend the planet. These bombs would devastate most of the planet and kill the majority of human life on the planet, whilst doubtless causing minimal damage to the invasion forces.

Invasion

An alien civilization would doubtless subjugate Earth within a matter of hours - there would be no long-term wars, no destruction of cities (which would seriously damage the environment and resources which the aliens supposedly desire), and no method of defending the planet. Alien invaders, realizing that humans have nuclear weapons which could cause irrevocable damage to their new planet, would doubtless disable all human technology from orbit, and would probably not even invade until all possible threats had been wiped out. Contrary to many works of science-fiction, most notably "War of the Worlds" , the aliens would not face a threat from Earth bacteria or viruses. Alien medical technology would be infinitely superior, and therefore capable of curing all diseases to be found on Earth. Considering that the alien species would most likely be unable to breathe our atmosphere, they would not even come into contact with the air.

It can only be conjectured what the alien invaders would do following their successful conquest. Many works of science fiction and popular culture, including films, books, and computer games, deal with the scenario of Earth following alien conquest. These range from human/alien co-operation to slavery and genocide. However, it is unlikely that any would prove to be realistic. It is more than likely that aggressive invaders would instantly eliminate the human species prior to settling into their new home.

Of course, the likelihood of aliens arriving in the vicinity of Earth in the near future is highly unlikely. It is far more likely that if and when an alien civilization - whether friendly or hostile - comes across Earth, it will be long after the human race has disappeared. A few relics may be left of human civilization, which will doubtless be destroyed in the future following war, disease, or ecological catastrophe, ensuring that aliens will land on a planet with a different dominant species, or none at all.

External link

• Alien Invasion at a site discussing threats (likely and unlikely) to the future of life on Earth