Toclafane invasion
Here... comes... the DRUMS!
The Toclafane invasion would have occured in the early 21st century on Earth, but due to the actions of the Tenth Doctor, Martha Jones and Jack Harkness, the invasion never took place. (see The Year that Never Happened)
History
Shall we decimate them? That sounds good, nice word, decimate. Remove one tenth of the population!
It began when The Master travelled to the year 100 Trillion and the location of the planet Utopia, to discover the remnants of humanity turning themselves into machines in a desperate effort to avoid the heat-death of the Universe. Taking a few of them back to Earth in The Doctor's stolen TARDIS, The Master tricked the citizens of Earth into believing they were a friendly race called the Toclafane. This was later revealed when the Toclafane/Humans killed the American President Coleman Winters, and using the space-time rift created by The Master's Paradox Machine (actually The Doctor's TARDIS) at exactly 8:02 descended upon the Earth.
Six billion Toclafane attacked all four corners of the Earth at once. Faced with sheer numbers and technology from the year 100 Trillion, human resistance was utterly crushed, being unable to even damage a Toclafane. Martha Jones personally witnessed the destruction of London following her escape from the Valiant.
However, one year later the Paradox Machine would be destroyed by Captain Jack Harkness, causing all the damage done by the Toclafane (aside from the death of President Winters and reporter Vivien Rook) to be undone. (DW: The Sound of Drums).
Everybody including the Toclafane forgot about it when the Paradox was destroyed, except for The Master, captain Jack, Martha Jones, her family, and of course The Doctor himself. (DW: Last of the Time Lords)
