The Manhattan Project was a secret U.S. government plan to obtain nuclear energy and use it to produce nuclear weapons. The program began in 1942 and ended in August 1945 after the successful use of two charges on the Japanese cities of Hiroshima and Nagasaki.
The Soviets did not have the data to produce nuclear weapons but received them from American scientists sympathetic to the Communists.
Just 12 days after the assembly of the first of the American bombs was completed, Theodore Hall and Klaus Fuchs, among others, gave the Soviets a number of documents and schematics. As a result, they did not have to develop the technology from scratch, but only improved American solutions.
Theodore Hall was a New York-born Soviet spy sympathetic to the communist Soviet Union. Klaus Fuchs was a German physicist who worked with the Main Intelligence Directorate (GRU) of the Soviet Union.
