World War III (WWIII or WW3) and the Third World War are names given to a hypothetical third worldwide large-scale military conflict subsequent to World War I and II. The term has been in use since at least as early as 1941. Some have applied it loosely to refer to limited or smaller conflicts such as the Cold War or the War on Terror, while others assumed that such a conflict would surpass prior world wars both in its scope and in its destructive impact.

Due to the development and use of nuclear weapons near the end of World War II and their subsequent acquisition and deployment by many countries, the potential risk of a nuclear devastation of Earth's civilization and life is a common theme in speculations about a Third World War. Another major concern is that biological warfare could cause a very large number of casualties, either intentionally or inadvertently by an accidental release of a biological agent, the unexpected mutation of an agent, or its adaptation to other species after use. High-scale apocalyptic events like these, caused by advanced technology used for destruction, could potentially make the Earth's surface uninhabitable.

Prior to the beginning of the Second World War, the First World War (1914–1918) was believed to have been "the war to end all wars," as it was popularly believed that never again could there possibly be a global conflict of such magnitude. During the interwar period, WWI was typically referred to simply as "The Great War." The outbreak of World War II in 1939 disproved the hope that mankind might have already "outgrown" the need for such widespread global wars.

With the advent of the Cold War in 1945 and with the spread of nuclear weapons technology to the Soviet Union, the possibility of a third global conflict became more plausible. During the Cold War years, the possibility of a Third World War was anticipated and planned for by military and civil authorities in many countries. Scenarios ranged from conventional warfare to limited or total nuclear warfare. At the height of the Cold War, a scenario referred to as Mutually Assured Destruction ("MAD") had been calculated which determined that an all-out nuclear confrontation would most certainly destroy all or nearly all human life on the planet. The potential absolute destruction of the human race may have contributed to the ability of both American and Soviet leaders to avoid such a scenario.

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