After Soviet Union tested their first atomic bomb, an arms race started with USA and USSR testing bigger and more powerful nuclear weapons. The nuclear arsenal increased rapidly. Serious worries about the destructive power on tap and accidents started to mount along with international outrage of increasing radioactive contamination of the environment. Having assured mutually assured destruction (MAD), USA and USSR started to re-evaluate the need of further development of nuclear weapons. Another concern was the number of nations that were in possession of nuclear weapons - nuclear proliferation.
Castle Bravo test by the Americans yield three times the expected yield and contaminated islanders and the Japanese fishing vessel Lucky Dragon