Alien invasion

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

Alien 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 and the Torchwood Institute developed defences against these attempts. (DW: Spearhead from Space, The Christmas Invasion)

Some of the most persistent species who attempted to invade the Earth include:
 * The Nestenes (DW: Spearhead from Space, Terror of the Autons, Rose)
 * The Cybermen (DW: The Invasion, The Tenth Planet, Silver Nemesis, The Wheel in Space, The Moonbase, etc)
 * The Daleks (DW: The Dalek Invasion of Earth, The Parting of the Ways, Doomsday, The Stolen Earth, etc)
 * The Ice Warriors (DW: The Seeds of Death, The Ice Warriors, NA: The Dying Days)

Other species that attempted to invade earth on various occasions include and many others.
 * The Kraal
 * The Bane
 * The Racnoss
 * The Sycorax
 * The Gelth
 * The Zygons

Most of these have been stopped 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 in Sarah Jane's a group of neighbors and friends) do their part in preventing alien invasions with Sarah Jane choosing not to interact with Torchwood due to her more peaceful nature. While these two groups have dealt with smaller threats, the bigger ones are too much for them and require the Doctor to deal with.

Human 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, such as at least half of Mutter's Spiral including Delta Magna (DW: The Power of Kroll) and the Althosian system (NA: The Pit), and also many of the worlds in neighboring galaxies such as the Andromeda Galaxy. (DW: The Ark in Space)

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