Truman reveals Soviet Union is now a nuclear power, Sept. 23, 1949

On September 23, 1949, President Harry Truman announced the evidence of the USSR's first nuclear device detonation. An excerpt from the article:

"On this day in 1949, President Harry S. Truman revealed that the Soviet Union had exploded an atomic bomb, ending the American monopoly in nuclear weapons years ahead of what was then thought possible by most U.S. officials and scientists.

Some 25 days before Truman’s announcement, U.S. monitoring stations had recorded seismic activity within the Soviet Union that carried the hallmarks of an underground nuclear test. At first, Truman doubted that such an event had occurred. He told his scientific advisers to recheck their data.

However, once the results were confirmed, the president, seeking to pre-empt an announcement by the Kremlin or a news leak, seized the initiative. He released a brief statement that read: 'We have evidence that within recent weeks, an atomic explosion occurred in the USSR.'”