Thirty-five years ago, Sir John Hackett published The Third World War, which speculated how WW3 might start and how it would be fought. Since it is now fashionable to call WW3 the Cold War, the time is right to publish a book about WW4, how it might start and most likely be fought. War planners must envision the unexpected and plan for the improbable, and the 20th century’s theories of total war are going to be rendered obsolete by the 21st century’s nuclear-enforced concept of limited war. In the future, with mutual acceptance of national survival in place, Mutually Assured Survival (MAS) wars will be waged between nuclear powers without introducing nuclear weapons.