World War III or the Third World War typically refers to a hypothetical worldwide conflict taking place at some point after the end of World War II. In fiction, the most common version of such a war is a nuclear conflict between the United States and the Soviet Union during the Cold War period.

The outbreak of such a war in January 1951 is the basis of the Harry Turtledove series The Hot War. The actual war may be found at World War III (The Hot War).

In addition, The Valley-Westside War has an apocalyptic Russian-American War with nuclear weapons be the relevant Point of Divergence. Moreover, several short works are implicitly set after a nuclear war between the USA and the USSR. In one alternate referenced in Gunpowder Empire (but not visited by the Crosstime Traffic POV characters), nuclear weapons were never invented, and the USA and USSR are fighting World War VI in the 2090s.

Atypically, a global nuclear war called the Third World War occurred as part of the back-story of In the Presence of Mine Enemies. In this war, the United States was pitted not against the USSR, but against longer-lived versions of Nazi Germany and Imperial Japan.