Alien invasion

An alien invasion is the process of any sapient species invading another planet, and attempting to dominate or eradicate the natives.

Invasions of Earth
Many extraterrestrial species tried to invade Earth from the 20th to the 22nd century, mainly because Humans were then a developing race, and by sending space probes out into space they attracted the attention of other, more developed races. However, various organisations such as UNIT, the Torchwood Institute and The Department (which was ironically created by aliens plotting to invade Earth) developed defences against these attempts. (DW: Spearhead from Space, The Christmas Invasion, K9TV: Regeneration)

Some of the most persistent species who attempted to invade the Earth include:
 * The Nestenes in the 1970s and 2005. (DW: Spearhead from Space, Terror of the Autons, Rose)
 * The Cybermen in the 1970s, 1986, 1988, some time in the 21st century and 2070. (DW: The Invasion, The Tenth Planet, Silver Nemesis, The Wheel in Space, The Moonbase, et al)
 * The Daleks attempted to invade Earth in the 1970s and 2007, began an attack in 200,100 and succeeded in 2157. (DW: Day of the Daleks, Doomsday, The Parting of the Ways, NA: GodEngine)
 * The Ice Warriors began an invasion in the 21st century, and succeeded in occupying Britain in 1997. (DW: The Seeds of Death, NA: The Dying Days)
 * The Korven failed in conquering Earth in 2480, starting a period of conflict with Humanity lasting centuries. (K9TV: The Korven) Then in the year 50000, Lomax manipulated 21st century history to weaken Earth and allow an invasion. This was also foiled. (K9TV: The Eclipse of the Korven)

Other species that attempted to invade Earth include:
 * The Bane (SJA: Invasion of the Bane)
 * Bandrils (NA: The Dying Days)
 * Drahvins (NA: The Dying Days)
 * The Gelth (DW: The Unquiet Dead)
 * The Kraal (DW: The Android Invasion)
 * Nimon (BFA: Seasons of Fear)
 * The Racnoss (DW: The Runaway Bride)
 * The Sycorax (DW: The Christmas Invasion)
 * Vardans (NA: No Future)
 * The Zygons (DW: Terror of the Zygons)
 * plus many other alien species.

Several (if not most) of these were prevented (for the most part) by the Doctor throughout various time periods, but in the 21st century groups led by former companions Sarah Jane Smith and Captain Jack Harkness (in his case, Torchwood 3, in Sarah Jane's a group of neighbors and friends) did their part in preventing alien invasions. Sarah Jane chose not to interact with Torchwood due to her dislike of Torchwood's weapons. While these two groups dealt with smaller threats, the larger invasion attempts were usually dealt with by the Doctor. (DW: Journey's End) Later in the 2050s, K9 Mark 2 formed a group of friends which safeguarded the Earth in the same fashion as Sarah Jane. (K9TV: 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 (DW: The Power of Kroll) and the Althosian system (NA: The Pit), and many of the worlds in neighboring galaxies such as the Andromeda Galaxy. (DW: The Ark in Space)

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

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