How did Soviet agents obtain U.S. atomic secrets during the early Cold War?

During the early Cold War, Soviet agents obtained critical U.S. atomic secrets primarily through a network of spies embedded within the Manhattan Project. This espionage was instrumental in accelerating the Soviet Union’s nuclear program, saving them significant time and resources.1

Key figures in this intelligence effort included:

• Klaus Fuchs: A German-born physicist working at Los Alamos, Fuchs provided the Soviets with detailed knowledge of atomic bomb theory and design. His contributions were vital to the development of the first Soviet plutonium device, which was a direct copy of the U.S. Fat Man bomb.21
• Theodore Hall: As the youngest physicist at Los Alamos, Hall passed technical details regarding the implosion device to Soviet intelligence through intermediaries.3
• Julius and Ethel Rosenberg: Julius Rosenberg, an engineer, coordinated an espionage ring that included Ethel’s brother, David Greenglass, a machinist at Los Alamos. Greenglass provided data on nuclear weapons, which the Rosenbergs passed to Soviet contacts.4

These activities, overseen by Soviet intelligence officials like Lavrenty P. Beria, allowed the Soviet Union to successfully test its first atomic weapon in 1949.1