2021 Sontaran invasion of Earth

The 2021 Sontaran invasion of Earth was a full-scale assault on Earth launched by the Sontarans in the aftermath of the Flux (TV: The Halloween Apocalypse) as foreseen by their Psychic Command. Settling on the planet on 31 October 2021, the Sontarans went back in time to the Crimean War and changed Earth's history, replacing the nations Russia and China with Sontar. The Sontarans of 1855 declared an alternate Crimean War against the British Army, which was overseen by the Sontarans of 2021, who made the city of Liverpool their primary base. The Sontaran invasion was ultimately thwarted and erased when Dan Lewis and Karvanista piloted a Sontaran timeship into a temporal collision with the rest of its fleet. (TV: War of the Sontarans)

Following this defeat, the Sontarans plotted their revenge alongside the Grand Serpent. The Serpent infiltrated UNIT from the inside in 1958, successfully maneuvering its shutdown in 2017 in preparation for the Sontarans' arrival. (TV: Survivors of the Flux) The Sontarans, who had been in conflict with the Daleks and Cybermen for control of the post-Flux universe, sought to offer their enemies an alliance that would combine the three empires' efforts to wage war on the rest of the universe. In secret, however, the Sontarans planned to betray the Daleks and Cybermen, leaving them in position for the newly returned Flux to destroy. Unknown to the Sontarans though, they had been outwitted by the Thirteenth Doctor, Eustacius Jericho and Claire Brown, and found themselves annihilated when the Flux struck again on 5 December, ending the Sontaran invasion of Earth once and for all. (TV: The Vanquishers)