"Hearts of Iron II: Doomsday is a stand-alone expansion for the game of World War II strategy, Hearts of Iron II. The new Doomsday scenario takes place two months after the end of World War II. On October 2, 1945, Soviet forces attack Allied forces in West Germany, Austria and South Korea. In response, US President Harry S. Truman orders the United States Air Force to drop atomic bombs on the Soviet capital, Moscow and Kuybyshev. This triggers World War III. The war continues into the 1950s and ends in 1953, at which point a winner is chosen."
Source: Wikipedia, "Hearts_of_Iron_II:_Doomsday," available under the CC-BY-SA License.