After World War II, Sweden considered building nuclear weapons to defend themselves against an offensive assault from the Soviet Union. From 1945 to 1972 the government ran a clandestine nuclear weapons program under the guise of civilian defence research at the Swedish National Defence Research Institute (FOA).
By the late 1950s the work had reached the point where underground testing was feasible. However, at this time the Riksdag prohibited research and development of nuclear weapons, pledging that research should be done only for the purpose of defence against nuclear attack.