Challenge: Prevent the Third World War between the United States and Soviet Union from occurring.
OOC: POD post World War 2, very preferably post 1950. So it can be in any decade and you guys have free reign to hammer out almost everything in this history. Bonus points if you avoid "If only we'd elected his holiness Reagan Christ", or demagoguery like that either in character or out (that likewise entails electing a Democrat in place of Reagan causing this war should it take place in the 80's. It can be a part of the universe but not the cause of WW3 because that wouldn't happen only only be ideological fodder.).