The correct answer is that the United States' use of atomic weapons against Japan motivated the Soviet Union to increase its efforts to develop an atomic bomb. United States and the Soviet Union were two main super powers in the world after the World War II, and they were two main counterparts in so called Cold War. It was a state of political and military tension after WW II between powers in the Western Block (USA and its NATO allies) and powers in Eastern Block (the Soviet Union and its allies in Warsaw pact).