Alien invasion

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

History
In December 1869, the Gelth attempted an invasion of Earth via the Cardiff Space-Time Rift to replace their bodies that were lost in the Last Great Time War. They were stopped by Gwyneth. (TV: The Unquiet Dead)

In 2005, the Nestene Consciousness invaded Earth, intending to use the planet as a food source following the loss of its protein planets. It was defeated by the Ninth Doctor and Rose Tyler. (TV: Rose)

On Christmas 2006, Earth was invaded by the Sycorax, who used blood control to hold a third of the human population hostage. They were obligated to leave the planet in peace once the Tenth Doctor defeated their leader in combat, only for their ship to be destroyed on the orders of Prime Minister Harriet Jones. (TV: The Christmas Invasion)

In 2007, the Battle of Canary Wharf began when the Cult of Skaro attacked the Cybermen within the Torchwood Tower; an army of five million Cybermen from a parallel Earth had arrived first in what the Tenth Doctor observed was "too late" to be an invasion but a victory. The battle ended when the Doctor and Rose pulled the millions of warring Daleks and Cybermen into the Void. (TV: Army of Ghosts/Doomsday)

Falling through time after the Last Great Time War, the Dalek Emperor perverted the Fourth Great and Bountiful Human Empire and amassing an army of Dalek of human origin, culminating in an invasion of Earth in the year 200,100. This invasion was opposed by the Ninth Doctor and ultimately ended when Rose Tyler used her power as Bad Wolf to wipe out the Dalek Fleet. (TV: The Long Game, Bad Wolf)

Alternate timelines
In an alternate timeline in which John Hart took control of the British Empire beginning in 1887, he used the resources of the Torchwood Institute to fight off numerous alien invasions of Earth. As he admitted to Jack Harkness, John did not do this "out of the goodness of [his]] heart" rather, but out of necessity as he was stranded on the planet since losing his Vortex manipulator to Jack. (AUDIO: The Death of Captain Jack)

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 21st century Dalek invasion 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 50,000, 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 2010s. (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: Unnatural Selection)
 * The Remoraxians (COMIC: In With the Tide)
 * The Saturnyns (TV: The Vampires of Venice)
 * The Monks (TV: Extremis, The Pyramid at the End of the World, The Lie of the Land)

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)

Tivoli was the most invaded planet in the galaxy. It was inhabited by one of the oldest civilisations in the galaxy who simply let invaders do as they wished. Their anthem was "Glory to Insert Name Here" and their cities were designed to be comfortable for the invading armies. (TV: The God Complex)

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)