Alien invasion

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

4th century
The Nimon contacted Sebastian Grayle in 305 AD and offered him immortality in exchange for his help in invading Earth. The Eighth Doctor and Charley Pollard stopped this attempt but Grayle gained his promised immortality and, over the next 1500 years, made many other attempts to help the Nimon. However, the Doctor and Charley were always there to stop him. (AUDIO: Seasons of Fear)

16th century
In 1580, the Saturnyns tried to sink Venice to make the city their new home. This plan was stopped by the Eleventh Doctor. (TV: The Vampires of Venice)

19th century
In 1868, the Animus planned to conquer Earth using Zarbi that were under its control. This plan was stopped when Ian Chesterton destroyed the Animus with a train. (COMIC: Unnatural Selection)

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)

20th and 21st centuries
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, Regeneration) In particular, the Thirteenth Doctor noted UNIT to be a "fundamentally vital protection for planet Earth against alien invasion". (TV: Resolution)

Early 20th century
In 1902, a Rutan scout attempted to summon the Rutan Battlefleet to invade Earth, intending to exploit its strategic position as a launch point for the Rutan Host's war with the Sontarans. The Rutan was thwarted by the Fourth Doctor and Leela. (TV: Horror of Fang Rock)

In 1930, the Cult of Skaro created a race of Human-Dalek hybrids in New York in an attempt to ensure the survival of the Dalek race. This plan was thwarted when the Tenth Doctor added a bit of Time Lord DNA into the Dalek's experiment, giving the Human-Dalek hybrids "that little bit of freedom". The hybrids turned on the Daleks, leaving only Dalek Caan left. Caan then destroyed the hybrids and left Earth. (TV: Evolution of the Daleks)

Late 20th century
In the London Event, the Great Intelligence attempted to invade Earth with Robot Yeti. They were stopped by the Second Doctor, Jamie McCrimmon, Victoria Waterfield and British Army forces led by Colonel Alistair Gordon Lethbridge-Stewart. (TV: The Web of Fear) Later, the Cybermen, aided by Tobias Vaughn, invaded Earth. They were stopped by the Second Doctor, Jamie McCrimmon, Zoe Heriot and a newly formed UNIT, led by now Brigadier Lethbridge-Stewart. (TV: The Invasion)

In the 1970s, Lethbridge-Stewart noted that there had been two attempts to invade the Earth since the formation of UNIT, and that both were stopped with the assistance of the Doctor. There was a policy decision not to inform the public. At this time, the Nestenes tried to invade Earth. They were stopped by a newly regenerated Third Doctor with the help of Liz Shaw, Lethbridge-Stewart and UNIT. (TV: Spearhead from Space) Later in that same decade, the Nestene tried to invade Earth again, this time helped by the Master. The Third Doctor and UNIT once again defeated them. (TV: Terror of the Autons)

In 1974, the Remoraxians tried to invade and occupy Earth by flooding it through artificial climatic changes. The Brigadier and other UNIT members were possessed by some Remoraxians set in their throats. Thanks to the Third Doctor, they were freed and the whole Remoraxian fleet was compelled to leave the planet. (COMIC: In With the Tide)

In the 1970s or 1980s (see UNIT dating controversy), the Zygons began the final phase of their invasion preparations by infiltrating the International Energy Conference. They were stopped by the Fourth Doctor and UNIT. (TV: Terror of the Zygons)

The Vardans, allied with Mortimus, tried to conquer Earth in 1976 by inhabiting electronic media through the Mediasphere. This affected humans with aggression and led to increased anarchism and terrorism. Mortimus planned to broadcast a pop concert to a billion viewers, allowing the Vardans to beam directly into humans and take them over. This invasion was defeated by the Seventh Doctor, who trapped the Vardan Army in a standing wave between a satellite and a station antennae before beaming them back to Varda. (PROSE: No Future)

In 1980, the Kraals attempted to invade Earth using android duplicates. They were stopped by the Fourth Doctor and Sarah Jane Smith. (TV: The Android Invasion)

In 1986, the Cybermen planned to make Earth their new home as their own planet was dying. A small group attacked an Antarctic tracking station in preparation for the invasion but were ultimately stopped by the First Doctor, Ben Jackson and Polly Wright. (TV: The Tenth Planet)

In 1987, the Cybermen again attempted to invade Earth in search for the Nemesis. Once they had the Nemesis, they planned to make Earth their new home. They were stopped by the Seventh Doctor, Ace, and the Nemesis itself. (TV: Silver Nemesis)

Sometime before 1997, the Bandrils attempted to invade Earth but were fought off by UNIT. UNIT also stopped a Drahvin invasion sometime before this year. (PROSE: The Dying Days)

In 1997, the Ice Warriors succeeded in occupying Great Britain. (PROSE: The Dying Days)

Early 21st century
As of 2004, the Brigadier claimed that UNIT was responsible for defeating over 200 alien invasions of Earth, including Axons, Cybermen, Zygons and Quarks. (AUDIO: The Coup)

In 2005, the Nestene Consciousness once again 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)

At Christmas that year, the Empress of the Racnoss attempted to revive her "children" who were trapped in the centre of the Earth. The Tenth Doctor stopped her by flooding her base and drowning the still-waking Racnoss. (TV: The Runaway Bride)

In July 2008, the Bane tried to invade Earth by using a new soft drink, Bubble Shock!, to control the human population. This plan was foiled by Sarah Jane Smith, Luke Smith, Maria Jackson and Clyde Langer. (TV: Invasion of the Bane)

Seeking to replace their barren homeworld, (AUDIO: Serenity) Cell 114 used sleeper agents to stage an attack in Cardiff. Though this attack was stopped by Torchwood Three, they were made aware that other agents were already on Earth. (TV: Sleeper) The remaining members of Cell 114 plotted to convert the human race into their own kind, only to be destroyed by Jack Harkness. (AUDIO: Serenity)

In 2009, the Daleks once again invaded Earth as part of their plot to activate their Reality Bomb. They were stopped by the Tenth Doctor and his companions. (TV: The Stolen Earth/Journey's End)

In the 2010s, Amy Pond suggested that the Shakri cubes were part of an alien invasion during the Year of the Slow Invasion. In fact, the Shakri intended to wipe out humanity, only to be stopped by the Eleventh Doctor. (TV: The Power of Three)

Zygons, who had been laying in wait since the 16th century, infiltrated UNIT, intending to use the organisation in their attempt at making Earth their new home. To stop Kate Stewart from destroying London to foil the Zygons' plans, the Tenth, Eleventh and War Doctors intervened, temporarily erasing the memories of everyone involved until a fair compromise could be made. (TV: The Day of the Doctor)

Banishing the Boneless to their own dimension, the Twelfth Doctor remained unsure as to whether they intended to invade, but determined their presence to be malicious. (TV: Flatline)

In 2017, the Monks took control of Earth by altering the minds of the population and making them believe that the Monks had been in charge since humanity's beginning. Over a period of six months, the Twelfth Doctor convinced the Monks that he was helping them while he made a plan to defeat them. It was Bill Potts, however, who ended the Monks' reign when she used their own technology to broadcast the image of her mother into the minds of the humans, breaking the Monks' control. (TV: The Lie of the Land)

Late 21st century
Sometime in the 21st century, the Ice Warriors attempted to invade Earth using Earth's own T-Mat system. The invasion was stopped when the Second Doctor tricked the incoming Ice Warrior fleet into flying into the Sun. (TV: The Seeds of Death)

In 2070, Earth's Moonbase was attacked by Cybermen who were planning on using it in yet another attempt to invade Earth. They were stopped by the Second Doctor, Ben Jackson, Polly Wright and Jamie McCrimmon. (TV: The Moonbase)

In 2079, the Cybermen intended to invade Earth again. They attempted to use a space station known as "the Wheel" to achieve this but were stopped by the Second Doctor. (TV: The Wheel in Space)

22nd century
In the mid-22nd century, the Daleks successfully invaded Earth, keeping their hold on the planet for several years. They began drilling to the core of the planet, planning on replacing the core with a propulsion system. This plan was thwarted when the First Doctor, Susan Foreman, Ian Chesteron, Barbara Wright and David Campbell helped destroy the mining operation. After this defeat, and the subsequent uprising of the remaining human population, the Daleks left Earth. (TV: The Dalek Invasion of Earth)

30 years later, in the 2190s, the Daleks reprised and updated their plan, with the help of the Monk, invading again and maintaining control over Earth until eventually thwarted by the Eighth Doctor, Lucie Miller and Alex Campbell. (AUDIO: Lucie Miller / To the Death)

25th century
The Korven failed in conquering Earth in 2480, starting a period of conflict with humanity lasting centuries. (TV: The Korven)

30th century
By the 30th century, Tobias Vaughn, in the thousand years since the first Cyberman invasion, took credit for protecting humanity from Daleks, Jullatii, Cybermen, Draconians, Chelonians, Ice Warriors, Sess, Kraals, Nestenes, Greld, and Zygons. (PROSE: Original Sin)

Far future
In the year 50,000, Lomax manipulated 21st century history to weaken Earth and allow an invasion. This was foiled by K9 Mark 2 and his friends. (TV: The Eclipse of the Korven)

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 created by the Black Guardian where the First Doctor never left Gallifrey, and became Lord President, numerous alien races invaded Earth, fighting amongst themselves over the planet as well as against the native Silurians and Sea Devils. This timeline was negated when the Seventh Doctor retrieved the Key to Time. (COMIC: Time & Time Again)

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)

In an alternate timeline in which the Earth was ravaged by a 100 year war during the 20th and 21st centuries, the Daleks took advantage of the weakened state of the remaining population and successfully invaded Earth. This timeline was negated by the Third Doctor after he realised that the guerrilla soldiers who travelled back in time to the late 20th century to stop the war, were actually the reason it happened in the first place. (TV: Day of the Daleks)

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)

Other references
The Ninth Doctor suggested that the Slitheen intended to invade Earth before learning that they themselves were not a species but a criminal family of Raxacoricofallapatorians who planned to exploit the planet for profit. (TV: World War Three)