Oppenheimer was suspected of treason. When the Soviet Union successfully tested an atomic bomb in 1949, it came as a surprise to the United States. Since it came so soon after the Manhattan Project, it was suspected that someone had betrayed the project.

One of the suspects was Oppenheimer, who was already believed to be a communist sympathizer. The Americans decided to respond to the Russian tests with a hydrogen bomb a thousand times more powerful. Oppenheimer initially opposed the project, but eventually agreed to participate. A successful hydrogen bomb test was conducted in 1952.