World War III was a limited, but global, nuclear and conventional conflict which took place in the early 21st century. It primarily involved wealthy and powerful developed countries whose destruction resulted in fundamental changes in the global balance of power, as long-established national boundaries and concentrations of population were broken.
Very little is known about the specific events that occurred at the turn of the new century, and thus the causes of WWIII. What is known is that the use of nuclear weapons was mercifully limited; the states worst hit by them were the United States and China, though the various European countries did not escape unscathed either. The city of Berlin was destroyed, for example.
As a result of the war the global geopolitical landscape has become heavily balkanized, with even formerly strong First World countries like the United States divided into smaller, less stable competitors. Civil wars and non-state revolutionary movements pose a constant security threat worldwide, and the millions of refugees displaced by the global tumult have become a major problem.
The North American continent is now dominated by, but not entirely under the jurisdiction of, the American Empire, which competed bitterly with the state of Mexico for land and resources south of the former borders of the United States.
Nuclear exchanges occurred in the Asian theater; however, the island nation of Japan never committed its military to actual combat. It emerged from the war politically intact, and now occupies a position of relative privilege. However, its capital city was destroyed by a nuclear strike; indeed, Tokyo Prefecture largely sank below sea level.
The "Japanese Miracle" - new technology which could scrub fallout and thus reduce the deterrent value of atomic weapons - contributed to Japan's great diplomatic clout in the postwar world, and the American Empire's relative weakness.
World War IV was a non-nuclear war (also known as the Second Vietnam War) triggered by the ensuing collapse of many Third World states.