Since nuclear weapons were conceived, their primary role has evolved in fits and starts in response to changes in the international security environment. They were originally pursued in a race to prevent Nazi Germany from acquiring them first in World War II. Some three months after victory in Europe, development of these weapons was completed and they were immediately used against Japan to shorten the Pacific War and prevent the large numbers of casualties anticipated in the planned invasion of the Japanese home islands. In the aftermath of Japan’s subsequent surrender, they were viewed as instruments of warfighting that enabled the United States to safely downsize its overseas military presence in the face of the Soviet Union’s superior conventional might that threatened the free nations of Western Europe.