Sontar was a nation on Earth that, according to Mary Seacole and General Logan, had "always existed" by 1855 in an alternate timeline created by the Sontarans who invaded Liverpool during the Flux. Its landmass was formed of what should have been Russia and China, within the Northern and Eastern Hemispheres, and it was the opposing faction during an alternate version of the Crimean War against the British troops.
The Sontarans, led by Commander Skaak, sought to reclaim Earth in honour of Commander Linx, (TV: War of the Sontarans) who had first laid claim to the planet long ago. (TV: The Time Warrior) Skaak chose the Crimean War as their first skirmish, both for its opportunity for bloodshed and because he wanted to ride a horse.
The timeline was later aborted when Dan Lewis and Karvanista piloted a Sontaran timeship into a temporal collision with the rest of its fleet in 2021 Liverpool, thus erasing the Sontarans' Flux assault from history. (TV: War of the Sontarans)