The tension between the two superpowers (i.e. America and USSR) began when there were rumors of a nuclear attack that would happen any moment. This was a period known in history as the Cold War, where espionage between the two superpowers were happening simultaneously. It only decreased when the UN, together with other nations, have agreed that such attack would be deemed a failure for both parties and a failure for humanity. Agreements and settlements regarding such issue was addressed properly and thus the Cold War faded and eventually completely ended.
