For the Players, an Endgame was a form of the Game where they would use all available resources a planet had to wage war. With the use of nuclear weapons, the planet could potentially be destroyed. The scoring system was also changed. Bonus points went to a Player who killed a long-time enemy. (PROSE: Endgame) Some Eternals engaged in similar games. (COMIC: Uninvited Guest)
Deciding that the time had come for an Endgame so that they could move on to other worlds, the Players attempted to start World War III in 1951 by plunging America and Russia into nuclear war; play would only end when one side annihilated the other. The Players used agents to influence the mind of Harry S. Truman in the White House, but were prevented when the Eighth Doctor informed the Secret Service of their plan.
In Russia, the Countess seduced Josef Stalin, and whispered in into his ear whilst he slept that China and America were aiming nuclear missiles at his cities. The Doctor convinced her to abort her game as she saw more potential to play further games on Earth, the Countess subsequently assisting him in stopping other Players' efforts to fully brainwash Truman. (PROSE: Endgame)