When was the first atomic bomb explosion ?
President Truman notified the world of the situation on September 23 , 1949 : " We have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R . " ( Truman 's statement likely in turn surprised the Soviets , who had hoped to keep the test a secret to avoid encouraging the Americans to increase their atomic programs , and did not know that the United States had built a test - detection system using the WB-29 Superfortress . )