The most common hypothetical scenario for World War III involves a nuclear war that is devastating to much of the Earth. Traditionally, this war has been imagined to occur between the United States and Russia, formerly the Cold War enemy of the United States when Russia was a part of the Soviet Union. Historically, the world came closest to an actual World War III during the Cuban missile crisis.
According to Weapons and Technology, another hypothetical scenario proposed for World War III is a battle between China and the United States, fought over economic and ideological factors in the Pacific Ocean and Southeast Asia. An expansion of this scenario allies China with the Islamic nations. A further scenario pits the countries of the West against China and Russia, with India joining the Western powers.
Not all hypothetical scenarios for World War III involve nuclear weapons. In 1947, the United States planned for Operation Dropshot, which assumed a conventional war would take place between the Soviet Union and the NATO forces, with nuclear weapons used only as a last resort.
In planning for a hypothetical World War III, countries establish continuity of government plans to make sure the government can continue if the head of government is killed.