World War III (WWIII or WW3 or the Third World War) is a term used to describe a worldwide conflict following World War II. The most common scenario, a hypothetical nuclear war between the superpowers, the Soviet Union and the United States, is widely used as a premise or plot device in books, films, television productions, and video games. However, some writers have applied the term instead to the Cold War, arguing that it met the definition of a world war even though there was no direct conflict between the superpowers.

World War I (1914–1918) was regarded as the "war to end all wars" and it was believed there could never be another global conflict. World War II (1939–1945) proved that to be false, and with the advent of the Cold War (1945–1991) and the use of nuclear weapons, the likelihood of a third global conflict became more accepted. It was anticipated and planned for by military and civil authorities in many countries. Scenarios ranged from conventional warfare, to limited or total nuclear warfare leading to the destruction of civilization.