The alien invasion is a common theme in science fiction stories and film, in which extraterrestrial life invades Earth to either exterminate and supplant human life, enslave it under a colonial system, harvest humans for food, or destroy the planet altogether.
The invasion scenario has been used as an allegory for a protest against military hegemony and the societal ills of the time. H.G. Wells' novel The War of the Worlds is often viewed as an indictment of European colonialism and its "gunboat diplomacy"—setting a common theme for some politically motivated future alien invasion stories. The theme also exploits invasion panics that were common when science fiction was first emerging as a genre.
Prospects of invasion tended to vary with the state of current affairs, and current perceptions of threat. Alien invasion was a common metaphor in US science fiction during the Cold War, illustrating the fears of foreign (e.g. Soviet Union) occupation and nuclear devastation of the American people. Examples of these stories include "The Liberation of Earth" by William Tenn and The Body Snatchers.
In the invasion trope, fictional aliens contacting Earth tend to either observe (sometimes using experiments) or invade, rather than help the population of Earth acquire the capacity to participate in interplanetary affairs. There have been a few exceptions, such as the alien-initiated first contact that begins the 1951 film The Day the Earth Stood Still, and the Vulcan-initiated first contact that concludes the 1996 film Star Trek: First Contact (although after a failed invasion by the Borg in the rest of the film). In both cases, aliens decide to visit Earth only after noticing that its inhabitants have reached a threshold level of technology: nuclear weapons combined with space travel in the first case, and faster-than-light travel using warp drive technology in the second.
Technically a human invasion of an alien species is also an alien invasion, as from the point of view of the aliens, humans are also aliens. Such stories are much rarer than aliens attacking humans stories. Examples include the 1989 video game Phantasy Star II, the 2007 film Battle for Terra, The Martian Chronicles by Ray Bradbury, the Imperium of Man in the Warhammer 40,000 universe, the 2009 movies Planet 51 and Avatar and the 2011 movie Mars Needs Moms.
As well as being a sub-genre of science fiction, these kinds of books can be considered a sub-genre of Invasion literature, which also includes fictional depictions of humans invaded by other humans (for example, a fictional invasion of England by a hostile France strongly influenced Wells' depiction of a Martian invasion).
In 1898 H.G. Wells published The War of the Worlds, depicting the invasion of Victorian England by Martians equipped with advanced weaponry. It is now seen as the seminal alien invasion story and Wells is credited with establishing several extraterrestrial themes which were later greatly expanded by science fiction writers in the 20th Century, including first contact and war between planets and their differing species. There were, however, stories of aliens and alien invasion prior to publication of The War of the Worlds.
In 1727 Jonathan Swift published Gulliver's Travels. The tale included a race of beings similar but superior to humanity, who are obsessed with mathematics. They live on a four-and-one-half-miles-in-diameter floating island fortress called Laputa, and use its shadow to prevent sun and rain from reaching earthly nations over which it travels, ensuring they will pay tribute to the Laputians.
Voltaire's Micromégas (1752) includes two aliens, from Saturn and Sirius, who are of immense size and visit the Earth out of curiosity. Initially they believe the planet is uninhabited, due to the difference in scale between them and human beings. When they discover the haughty Earth-centric views of Earth philosophers, they are very much amused by how important Earth beings think they are compared to actual titans such as themselves.
In 1892 Robert Potter, an Australian Clergyman, published The Germ Growers in London. It describes a covert invasion by aliens who take on the appearance of human beings and attempt to develop a virulent disease to assist in their plans for global conquest. It was not widely read, and consequently Wells' vastly more successful novel is generally credited as the seminal alien invasion story.
Wells had already proposed another outcome for the alien invasion story in The War of the Worlds. When the Narrator meets the artilleryman the second time, the artilleryman imagines a future where humanity, hiding underground in sewers and tunnels, conducts a guerrilla war, fighting against the Martians for generations to come, and eventually, after learning how to duplicate Martian weapon technology, destroys the invaders and takes back the Earth.
Six weeks after publication of the novel, the Boston Post newspaper published another alien invasion story, an unauthorized sequel to The War of the Worlds, which turned the tables on the invaders. Edison's Conquest of Mars was written by Garrett P. Serviss, a now little-remembered writer, who described the famous inventor Thomas Edison leading a counterattack against the invaders on their home soil. Though this is actually a sequel to Fighters from Mars, a revised and unauthorised reprint of War of the Worlds, they both were first printed in the Boston Post in 1898
The War of the Worlds was reprinted in the United States in 1927, before the Golden Age of science fiction, by Hugo Gernsback in Amazing Stories. John W. Campbell, another key editor of the era, and periodic short story writer, published several alien invasion stories in the 1930s. Many well known science fiction writers were to follow, including Isaac Asimov, Arthur C. Clarke, Clifford Simak, plus Robert A. Heinlein who wrote The Puppet Masters in 1953.
The most well-known alien invasion scenarios involve the aliens landing on Earth, destroying or abducting people, fighting and defeating Earth's military forces, and then destroying Earth's major cities. Usually, the bulk of the story follows the battles between the invaders and Earth's armies, as in The War of the Worlds. However, not all alien invasion stories follow this plot. In some accounts, the alien invaders will covertly subvert human society using disguises, shapechanging, creating conflict in humanity and let humans destroy themselves, or human allies. In other depictions, the aliens score an overwhelming victory over humanity and the bulk of the story occurs after the aliens have taken over. Sometimes, the aliens do not come from space, but from another dimension. And in some fiction, the invaders may not actually be aliens, but demonic creatures.
This is a familiar variation on the alien invasion theme. In the infiltration scenario, the invaders will typically take human form and can move freely throughout human society, even to the point of taking control of command positions. The purpose of this may either be to take over the entire world through infiltration (Invasion of the Body Snatchers), or as advanced scouts meant to "soften up" Earth in preparation for a full-scale invasion by the aliens' conventional military (First Wave). This type of invasion usually emphasizes common fears during the Cold War, with the Communist agents suspected everywhere, but has also become common during any time of social change and unrest. The classic examples of this would be Invasion of the Body Snatchers, the gradual evolution of humans to ‘hybrid’ aliens in TV's Invasion, Threshold, the Animorphs series, Invader Zim, Robert A. Heinlein's The Puppet Masters and the John W. Campbell, Jr. short story "Who Goes There?", which was made into 1951 Howard Hawks film The Thing from Another World, with a more faithful adaptation being made by John Carpenter in 1982 as The Thing. Another film by John Carpenter "Village of the Damned", based on John Wyndham's "The Midwich Cuckoos" shows aliens living on Earth whilst masquerading as human children. This also happens in the Doctor Who Series One episodes "Aliens of London" and "World War 3". Aliens could also control people to do their will through electronic devices implanted in human bodies such as in 1953's Invaders from Mars. Another example of alien infiltration is in the 2005 video game "Destroy All Humans!" in which the player controls an alien scout in order to destroy and eventually become the President of the United States, as well as annihilate humans in their path. In some cases, the aliens already look much like Humans - in the Star Trek franchise, in the 133 years before their participation in first contact in 2063, no fewer than six Vulcans spent significant amounts of time on Earth (though none were "invaders" - three were shipwreck survivors, the other half time travelers, in the company of at least one Human, from a future where their respective worlds were political allies), and were able to pass themselves as Human simply by using hair or hats to conceal their pointed ears; likewise the Bajoran Kira Nerys needed only to put a Band-Aid on her ridged nose, while at the same time the Trill Jadzia Dax was able to pass off her natural spots as tattoos.
This is a theme that can occur in many invasion stories. In short, the alien invaders win and occupy the Earth or human civilization or they simply try to terraform Earth to make it suit them better, as in The War Against the Chtorr (a more accurate term for this would be "xenoform"), at least until a human resistance movement overthrows the aliens and/or their puppet governments. Many occupation stories are influenced by the real human invasions by totalitarian governments, in which the alien invaders support existing human government infrastructures that welcome their new alien overlords or purge opposition governments and rebuild them in their own image and the enforcement of their rule through the use of collaborators and secret police. Examples of life under alien occupation can be seen in the TV series V, John Christopher's book series, The Tripods, the comic book miniseries Slash Maraud, and the Half-Life and also the video game series Resistance. In Nemo Ramjet's All Tomorrows the Qu - a superior alien race - after conquest of humanity reduce it to animal status by means of genetic engineering, making it impossible for resistance to occur. In the fictional history of the BBC science fiction programme Doctor Who, the genocidal Dalek race is heavily involved in invasion stories in the show, most recently the universal Armageddon story "Journey's End". That story especially includes Daleks conquering the earth and then occupying it along with other stolen worlds to initiate their stratagem for universal destruction. The recent film Battle: Los Angeles depicts Aliens coming to Earth supposedly for water.
These are short-term alien invasions by creatures incapable of supporting a large-scale invasion due to small numbers who instead use the shock of their arrival to inspire terror. Other stories following this line of reasoning would have the alien invaders conducting reconnaissance and probing raids on the Earth's population and especially their military forces. Also, the invaders will try to choose isolated spots, such as the desert or farmlands of rural zones in the United States, as a staging area or landing zone. This type of plotline provides a better possibility of small groups, like local police and military, or even ordinary civilians, having the ability to repulse the invaders and return to normal life after the event. Because of budget constraints, this variation was fairly common in the 1950s science fiction B-movies, such as It Came from Outer Space, Teenagers from Outer Space, The Blob, and Plan 9 from Outer Space.
This theme has also been explored in fiction on the rare occasion. With this type of story, the invaders, in a kind of little grey/green man's burden, colonize the planet in an effort to spread their culture and "civilize" the indigenous "barbaric" inhabitants or secretly watch and aid earthlings saving them from themselves. The former theme shares many traits with hostile occupation fiction, but the invaders tend to view the occupied peoples as students or equals rather than subjects and slaves. The latter theme of secret watcher is a paternalistic/maternalistic theme. In this fiction, the aliens intervene in human affairs to prevent them from destroying themselves, such as Klaatu and Gort in The Day the Earth Stood Still warning the leaders of Earth to abandon their warlike ways and join other space-faring civilizations else that they will destroy themselves or be destroyed by their interstellar union. Other examples of a beneficial alien invasion are Gene Roddenberry's The Questor Tapes movie and his 1968 Star Trek episode Assignment: Earth, Arthur C. Clarke's Childhood's End, the anime and novel series Crest of the Stars and David Brin's Uplift series of books.
The little grey/green man's burden idea was commonly explored by Harry Turtledove's Worldwar and Colonization series. The reptilian Lizard-men that invade Earth have an imperialist mindset comparable in the narrative to 19th century models of cultural imperialism. One of the moral dilemmas of the series, however, is that the Lizards are consistently portrayed as morally superior to Nazi Germany of the 1940s, which they are trying to invade along with the other Axis and Allied powers. The Lizards actually liberate Treblinka, and ethnic groups in Eastern Europe being oppressed by the Nazis hail them as liberators. They have difficulty conceptually understanding why the Nazis are committing ethnic genocide, and on the whole, consider humanity to be so barbaric and warlike that they would benefit from being united under their culture.
In which the invaders are supernatural or otherwise religious-inspired demonic beings, who infiltrate the Earth, attack mankind, take over human society (disguised as humans themselves) and make war upon the saints, fulfilling the events described in the Book of Revelation or another religious prophecy, occasionally invented for the story itself. Warhammer 40,000 and The Doom computer game series follows this concept. The novel Childhood's End may be viewed as a form of demonic alien invasion, because of the Overlords' devilish appearances.
A period of the recent or distant past serving as the scene of an alien invasion of one of the aforementioned types. The most ambitious project of this kind seems to be Harry Turtledove's alternative history Worldwar & Colonization series, where lizard-like aliens land on Earth in 1942, bent on conquest, forcing the opposing sides of the Second World War to sign hasty cease-fires and fight their own (largely) separate wars against the invaders. In Sideslip by Ted White and Dave van Arnam, a private detective from our New York finds himself in an alternate reality where Earth is under occupation by interstellar humanoids nicknamed "Angels", who had landed in 1938, taking advantage of the confusion following Orson Welles' War of the Worlds radio program, and had ruled Earth as a colony ever since. In Starspawn by Kenneth Von Gunden, Earth is infiltrated by small parasitic aliens capable of attaching themselves to a human and controlling him or her - similar to the scenario of Heinlein's aforementioned The Puppet Masters - except that the invasion takes place in Medieval England, against the background of knights besieging a castle. In similar settings at Poul Anderson's The High Crusade, an alien ship lands at a Medieval English village, but the overconfident would-be conquerors find out the hard way that they are not immune to swords and arrows; the humans take over the ship and proceed to carve out an empire among the stars, but lose contact with Earth which goes on with its familiar history. In Eifelheim by Michael Flynn, an alien spaceship lands in central Europe in the middle of the Black Death. In Doctor Who, because of the time travel element, invasions happen a lot, most notably the cyber-man invasion of Victorian London, using a 60m tall cyber-king walker, and the Jon Pertwee episode depicting an invasion of a small village by The Master. The Stargate franchise is based on this concept, where an alien race visited Earth and took on the mantle of ancient gods with the intent of capturing different groups of humans and sending them elsewhere to use as slave labor or corporal hosts. Much of the television program Stargate: SG-1 dealt with meeting descendants of such humans who had been transported across the galaxy and whose technological development evolved along different paths, as well as preventing the malicious aliens from attacking Earth. The 1996 movie Star Trek: First Contact deals extensively with this theme, although the frame of reference is in the future; the Borg come to Earth in 2063, approximately two to three hundred years prior to the relevant events in the Star Trek universe. This concept has also been explored outside of the film and novel forms of entertainment. The storyline of the PlayStation 3 hit series Resistance (first developed in 2006 and still in development today), where Earth had been invaded by an alien race that uses microbes to change the DNA of earth's inhabitants and biologically assimilates them into their hive-like society. From the point of invasion in the 1950s, Earth is spun into an alternate history setting that has yet to reach the date of 1960. A two minute video of the story so far can be found on YouTube as part of the official PlayStation Channel. Recent media, such as the Crysis series of games, involve an alien race that has survived (possibly in suspended animation) underground for a very long time, and has only recently emerged to take over Earth in the context of the story. This is also a theme in the Transformers film franchise, though the motivations of the Cybertronian robots do not necessarily involve conquest.
Occasionally, two or more themes can be used as a combination. For example, the aliens may first infiltrate society secretly, then, after gaining human trust, they will suddenly begin destroying Earth's cities, with the humans taken by complete surprise. Another example of this is in two episodes of the popular sci-fi show Stargate SG-1 an alien race known as the Aschen befriend humans and share their advanced technology and medicine freely in exchange for stargate addresses. But it soon becomes clear that the Aschen plan to eradicate the human race slowly by making both women and men infertile so the human race dies out over generations. Another type of invasion is seen in various Godzilla films, most notably Destroy All Monsters, and also in the Monster Wars saga of Godzilla: The Series and the 2004 film, Godzilla: Final Wars. In these films, alien races take control of earth's monsters and use them to attack and destroy Earth's major cities, but are usually ironically defeated by the monsters themselves.
The classic treatment was The War of the Worlds by H. G. Wells, which was made into movies in 1953 and 2005, as well as numerous radio adaptations and a TV series. Another early version is The Germ Growers (1892), by Robert Potter. Other treatments have posited biological invasions (Invasion of the Body Snatchers), or cultural invasion (The Uplift Wars by David Brin).
The 1988 cult film They Live uses its own alien infiltration back story as a satire on what some perceived as Ronald Reagan's America and the 1980s as an era of conspicuous consumption, in which the hidden aliens and human members of the elite oppress poverty-stricken humans and a shrinking middle class.
In Doctor Who, alien invasions are sometimes the centerpiece of Earth-set episodes.
The beginning of Half-Life 2 reveals an Earth that was invaded and overrun by a hostile alien empire, the Combine, via a portal. The game's protagonist later joined the human resistance effort.
The beginning half of the popular video game Halo 3 deals with the defense of Earth against a genocidal alien empire, the Covenant. The protagonist of the game along with Earth's military force, the UNSC, and a group of aliens who support the humans, the Covenant Separatists, eventually repel the invasion and topple the Covenant.
In the A.R. Curtis short story Best an invasion is portrayed more as a refuge accident, where aggressive cast-offs fight for supremecy against local police and civilians.
Planetary defense against invasion is a recurring theme in science fiction, usually designed to repel an invasion of a planet by an external force; human or otherwise. An example would be during the Earth Alliance Civil War on the television series Babylon 5, whereby the loyalist forces used orbital planetary defense platforms to defend Earth from the separatist forces.
Planetary defenses are not limited to one pre-defined type, some are space or orbital based platforms or ships as in the previously stated example. Another example of planetary defense platforms is Cairo station, which is from the Halo 2 game and the Halo novels. As well as serving as docking stations for UNSC ships, MAC stations such as Cairo Station also possess a massive coilgun, which serve to ward off assaulting enemy fleets via long-range bombardments. UNSC worlds often possess multiple MAC stations in orbit, to provide overlapping fields of fire against either human or alien invaders. There are also weapon systems based on the surface of planets occasionally, as in Stargate SG-1 where the hero uses an Ancient weapon in Antarctica to destroy the villain's invading fleet from the ground; others are simpler, including ground based fighters, as in the case in Star Wars Episode IV: A New Hope where single-manned fighters defended a rebel base against a moon-sized Death Star. In addition, in the novel The Armageddon Inheritance from the author David Weber's Empire From the Ashes trilogy, Antarctica possesses a core-tap generator, which yanks large energy reserves from hyperspace and projects it around Earth as a planetary shield. While serving to ward off enemy fleets and bolster Earth's defenses against most conventional assaults, it proved insufficiently powerful against large asteroids and small moons, as when in the book the moon Iapetus was hurled at Earth to "crack it open like a bullet through butter." A similar shield is used to defend a rebel base in Star Wars Episode V: The Empire Strikes Back, but it was too small to cover the entire planet. As a result, while it was powerful enough to withstand the Imperial fleet's orbital bombardment, it was overcome by landing ground troops who destroyed the shield generator. However, when combined with the planetary defense platforms in place, planetary shields can be very necessary to planet-wide protection. Some planetary defenses in science fiction are less fantastical than often proposed. For instance, the BBC television series Doctor Who has spawned two such examples, the fictional military organization UNIT stationed terrestrially to defend modern earth against invasion and a more secretive group Torchwood, who defend the United Kingdom against alien incursions. UNIT has appeared multiple times in Doctor Who when drastic events are involved, whilst Torchwood is the focus of a successful spin-off series.
The Legacy of the Aldenata series by John Ringo describe in detail a planetary defense scenario using alien technology to construct a fleet of starships. Similar themes can be seen in the Japanese anime series The Super Dimension Fortress Macross and its Robotech American adaptation. Both Macross and Robotech also include a "Grand Cannon" massive energy weapon based in the northern hemisphere that is used for planetary defense. Doc Travis S. Taylor's Warp Speed and The Quantum Connection books (known as The Warp War Series) gives examples of developing planetary defense systems based on warp technologies. The Von Neumann's War series by John Ringo and Travis S. Taylor also develops an organization designed to develop planetary defense strategies from present day technologies.