World War I was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. One of the deadliest conflicts in history, an estimated 9 million combatants and 7 million civilians died as a direct result of the war.

World War II was a global war that lasted from 1939 to 1945. A state of total war emerged, directly involving more than 100 million people from over 30 countries. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.

World War III is the name given to a hypothetical third worldwide large-scale military conflict subsequent to World War I and World War 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 have operated under the assumption that such a conflict would surpass both prior world wars in both the level of its widespread scope and of its overall destructive impact.

As of January 2019, World War III does not appear to have started - but there have been a number of historical close calls that are widely regarded as having been narrowly-missed possible catalysts of such a conflict.

In 1949, after the unleashing of nuclear weaponry at the end of WWII, physicist Albert Einstein suggested that any outcome of a possible WWIII would be so dire as to revert mankind back to the Stone Age. When asked by journalist Alfred Werner what types of weapons Einstein believed World War III might be fought with, Einstein warned, "I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones". It can be inferred here that Einstein assumed that World War III would either exterminate, or else nearly exterminate the human race, presumably due to nuclear warfare.