World War Three is a term used to describe a hypothetical future conflict of World War II–scale. Most usages of the term include the use of weapons of mass destruction such as nuclear weapons.
In the latter half of the 20th century, military confrontation between the two superpowers was considered to pose an extreme threat to establishing world peace, when the Cold War saw the capitalistic United States face the communistic Soviet Union. If this confrontation had escalated into full-scale war, it was widely thought that the conflict would become "World War III," and that the end result would be the destruction of most life on Earth, an extermination of human life or, at the very least, the partial collapse of civilization, with total casualties over 500 million. (See also Mutually Assured Destruction.) This outcome ranks with asteroid or comet impact events, worldwide pandemics, and catastrophic climate change as one of the major mass extinction events that could befall humanity or even all life on Earth.
The term has carried on beyond the Cold War, and now usually refers to any potential future global conflict which would involve nuclear weapons. In modern times, the possibility of WWIII taking place between superpowers has been replaced by the threat of a nuclear attack by a smaller party, which could incite retaliation and cause a destructive domino effect.