Alien invasion

An alien invasion was the process of any sapient species invading another planet and attempting to conquer or eradicate the natives.

Invasions of Earth
Many extraterrestrial species tried to invade Earth from the 20th to the 22nd century. Humans were a developing race; by sending space probes into space, they attracted the attention of more developed races. Organisations such as UNIT, the Torchwood Institute and the Department (which was created by aliens plotting to invade Earth) developed defences against these attempts. (TV: Spearhead from Space, The Christmas Invasion, TV: Regeneration)

Some of the most persistent species who attempted to invade the Earth include:
 * The Nestenes in the 1970s (TV: Spearhead from Space, Terror of the Autons) and 2005. (TV: Rose)
 * The Cybermen in the 1970s, (TV: The Invasion) 1986, (TV: The Tenth Planet) 1988, (TV: Silver Nemesis) some time in the 21st century (TV: The Wheel in Space) and 2070. (TV: The Moonbase)
 * The Daleks attempted to invade Earth in 1930, (TV: Evolution of the Daleks) the 1970s (TV: Day of the Daleks) and 2007; (TV: Doomsday) began an attack in 200,100; (TV: The Parting of the Ways) and succeeded in 2009 (TV: The Stolen Earth) and 2157. (PROSE: GodEngine)
 * See also Dalek Invasion of Earth (2009) and 22nd century Dalek invasion.


 * The Ice Warriors began an invasion in the 21st century, (TV: The Seeds of Death) and succeeded in occupying Britain in 1997. (PROSE: The Dying Days)
 * The Korven failed in conquering Earth in 2480, starting a period of conflict with humanity lasting centuries. (TV: The Korven) In the year 50000, Lomax manipulated 21st century history to weaken Earth and allow an invasion. This was also foiled. (TV: The Eclipse of the Korven)
 * The Zygons attempted to invade in the 1970s (TV: Terror of the Zygons) and in the early 21st century. (TV: The Day of the Doctor)

Other species that attempted to invade Earth include:
 * The 456 (TV: Children of Earth)
 * The Bane (TV: Invasion of the Bane)
 * The Bandrils (PROSE: The Dying Days)
 * The Drahvins (PROSE: The Dying Days)
 * The Gelth (TV: The Unquiet Dead)
 * The Kraal (TV: The Android Invasion)
 * The Nimon (AUDIO: Seasons of Fear)
 * The Racnoss (TV: The Runaway Bride)
 * The Sycorax (TV: The Christmas Invasion)
 * The Vardans (PROSE: No Future)
 * The Master Brain (TV: The Mind Robber)
 * The Zarbi (COMIC: Prisoners of Time)
 * The Remoraxians (COMIC: Prisoners of Time)
 * The Saturnyns (TV: The Vampires of Venice)

Several (if not most) of these were prevented (for the most part) by the Doctor in various time periods, (TV: The Dalek Invasion of Earth, The Invasion, The Unquiet Dead, The Christmas Invasion) but in the 21st century a group of neighbours and friends led by the Doctor's former companion Sarah Jane Smith did their part in preventing alien invasions. (TV: Invasion of the Bane, Eye of the Gorgon, Secrets of the Stars, The Temptation of Sarah Jane Smith et al.) While this group dealt with smaller threats, the larger invasion attempts were usually dealt with by the Doctor. (TV: Journey's End) Later in the 2050s, K9 Mark 2 became part of a group which safeguarded the Earth in the same fashion as Sarah Jane. (TV: K9)

Invasions of other planets
During the human colonisation period of the mid-3rd millennium to the 51st century and beyond, humanity invaded various planets for their empires, including at least half of Mutter's Spiral, including Delta Magna (TV: The Power of Kroll), the Althosian system (PROSE: The Pit) and many of the worlds in neighbouring galaxies such as the Andromeda Galaxy. (TV: The Ark in Space) The Vardans and Sontarans attempted to invade Gallifrey, the Doctor's home planet. (TV: The Invasion of Time)

Motives
There were many motives for alien invasion. Some species invaded due to the destruction of their homeworld, (TV: Terror of the Zygons, Rose, The Parting of the Ways, Invasion of the Bane, The Day of the Doctor, et al) or the rendering of their homeworld uninhabitable, (TV: The Tenth Planet, The Seeds of Death, et al) whilst others invaded to expand their empires. (TV: The Christmas Invasion, et al) One particular case had the Earth invaded as part of a plan to power a weapon. (TV: The Stolen Earth / Journey's End)

The Sontaran Commander Kaagh believed the common reason was that humans had 'no guts', remarking that it was 'no wonder' that half the species from Earth to the Verlixix Spiral Cluster had plans to invade the planet. (TV: Enemy of the Bane) The Twelfth Doctor, after hearing from a human that there was a horror movie called Alien, commented that with that 'really offensive' attitude it was 'no wonder everyone keeps invading you.' (TV: Last Christmas)

Some unknown power, organisation, or person had classed invasions by type. A type F invasion was said to be aesthetically motivated. (PROSE: The Well-Mannered War)