Proliferation was not only a numbers issue. As the weapons developed in sophistication from those dropped in Japan they became many orders of magnitude more destructive, representing a grave threat to humankind as a whole. By the early 1960s, nuclear weapons had been built that could cause devastation for hundreds of kilometres beyond the impact zone. The United States and the Soviet Union, who were locked into a system of rivalry known as the Cold War, seemed to be in a race to outdo each other in terms of the quantity and quality of bombs each possessed.
