The Platonic War was fought around the 347th century between humans and several alien species. It began after the failure of the peace conference of Anselm and ended with the victory of the humans.
For many years afterwards, there was resentment towards humans. The Slitheen family took advantage of this with their time travelling tourist agency. One of their entertainments was having native humans dress up as aliens to be slaughtered by other humans. Their secret plan for the tourist agency was to change the past, stopping human civilisation from ever starting. (PROSE: The Slitheen Excursion)