Alien invasion is a common theme in science fiction stories and film, in which an extraterrestrial society invades Earth with the intent to exterminate and replace human life , enslave it under a colonial system, to harvest humans for food, or sometimes to destroy the earth itself.

The invasion scenario has been used as an allegory for a protest against military hegemony and the societal ills of the time. Wells' "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.

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. 

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.  Aliens decide to visit Earth only after noticing that its inhabitants have reached a threshold level of technology: nuclear weapons combined with space travel, and faster-than-light travel using warp drive technology are somee examples.

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. 

As well as being a sub-genre of science fiction, these kind 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).

Types of Alien Invasion and theories

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.

Alien infiltration has been 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. This type of invasion usually emphasizes paranoid fears and was very common during the Cold War, with the Communist agents suspected everywhere, but has also become common in during any time of social change and unrest. 

Alien occupation can occur in many invasion stories. In short, the alien invaders win and occupy the Earth or human civilization (sometimes they even try to terraform the earth to make it suit them better [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, such as Nazi Germany, 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. After conquest of humanity the goal would be to reduce it to an animal status by means of genetic engineering, making it impossible for resistance to occur.

Alien raids are short-term alien invasions. The aliens are incapable of supporting a large-scale invasion due to small numbers and 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, the ability to repulse the invaders and return to normal life after the event. 

Beneficial alien invasion 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.

Another conception of the alien invasion theme is a demonic alien invasion, 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.

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.