Answer: Soviet Union
Explanation: In the Soviet Union, interest in nuclear weapons began with the discovery of fission in 1938, in Germany where Nazi physicists worked. Already in the following 1939, Soviet scientists attempted to carry out the same experiment, when it began to examine conditions that lead to a nuclear reaction. It was interrupted during the WWII due to war conditions.
However, after the end of WWII and atomic attacks on Japan by America, the Soviets intensified their nuclear research. Since the Americans showed that they already possess atomic weapons and that tensions began between the USSR and America immediately after WWII, which would lead to Cold War, the Soviets in December 1946 managed to create the first nuclear chain reaction. The first nuclear reactor was made in 1948, while the first nuclear device was tested in 1949. Further during the decade of the Cold War, there was a steady development of nuclear weapons.