Multiple spies working inside the American and British nuclear weapons programs passed atomic secrets to the Soviet Union during and after World War II. The most damaging were Klaus Fuchs, a German-born physicist working at Los Alamos, and Theodore Hall, a teenage prodigy recruited to the Manhattan Project. They were supported by a wider network of couriers and lower-level sources, including David Greenglass, Julius Rosenberg, and George Koval. Together, their espionage shaved at least two years off the Soviet timeline for building its own bomb.
Klaus Fuchs: The Most Damaging Spy
Klaus Fuchs was a German-born theoretical physicist who fled Nazi Germany in the 1930s and eventually became a British citizen. He worked on the British atomic research program before being sent to Los Alamos as part of the joint Allied effort to build the bomb. While there, he provided information on technical issues relating to the design of the atomic bomb to Soviet military intelligence, known as the GRU. His access was extraordinary: he worked on the mathematical models for the implosion method used in the plutonium bomb, giving him deep knowledge of how the weapon actually functioned.
Fuchs was a committed communist who volunteered his services. He began passing secrets before he even arrived at Los Alamos, sharing British atomic research with a GRU contact. Once inside the Manhattan Project, the quality and detail of his information increased dramatically. The Soviets used his detailed design descriptions to build a near-copy of the Fat Man plutonium bomb. That replica, which Western intelligence dubbed Joe-1, was detonated at the Semipalatinsk test site in Kazakhstan on August 29, 1949, with a yield of about 22 kilotons. Russian scientists later admitted that Fuchs’s contributions sped up their nuclear program by at least two years.
Theodore Hall: The Teenage Spy
Theodore Hall was just 18 years old when he arrived at Los Alamos in 1944, one of the youngest scientists on the project. A physics prodigy who had entered Harvard at 14, Hall made contact with Soviet intelligence in November 1944 on his own initiative. Like Fuchs, he was motivated by ideology rather than money. He believed the United States should not hold a monopoly on nuclear weapons.
Hall provided detailed drawings and measurements of the plutonium bomb, including information about the implosion mechanism that was central to the weapon’s design. His contributions proved integral to the Soviet development of nuclear capabilities. Unlike nearly every other atomic spy, Hall was never prosecuted. U.S. intelligence identified him through decoded Soviet cables but lacked enough admissible evidence for a trial. He lived quietly as a biophysics researcher in England until his death in 1999.
The Rosenberg Network
Julius Rosenberg ran a small espionage ring that included his brother-in-law, David Greenglass, a machinist stationed at Los Alamos. Greenglass did not have the scientific depth of Fuchs or Hall, but he had hands-on access to bomb components. He sketched diagrams of the high-explosive lens molds used in implosion research and, by his own testimony, drew up a sketch of the atom bomb itself along with descriptive material explaining it.
The lens mold sketches illustrated a critical principle: how combinations of explosives with different detonation speeds could be shaped to focus a shockwave inward toward a common center. At trial, a prosecution witness confirmed that while the sketches lacked precise dimensions, they conveyed enough information for someone familiar with the field to understand the idea behind the experiment. Greenglass handed these materials to Harry Gold, a courier who also collected information from Fuchs, receiving an envelope of cash in return.
Julius and Ethel Rosenberg were convicted of conspiracy to commit espionage and executed in 1953. Their case became one of the most polarizing events of the Cold War. Greenglass, who testified against his sister Ethel, later expressed regret, saying he had been “young, stupid, and immature, but a good Communist.” Decades later, newly released evidence suggested Ethel’s involvement was far more limited than prosecutors had claimed at trial.
George Koval and the Bomb’s Trigger
One of the least-known atomic spies was George Koval, a Soviet military intelligence officer who had been born in the United States to Russian immigrant parents. Koval was drafted into the U.S. Army and, through a combination of luck and placement, ended up working at several Manhattan Project facilities. In June 1945, he was transferred to a top-secret laboratory in Dayton, Ohio, where scientists were developing the polonium-based initiator for the implosion bomb. This was the tiny device at the very core of the weapon that triggered the nuclear chain reaction.
This may have been his most damaging placement. Between 1945 and early 1946, Koval met regularly with his Soviet contact to pass along detailed information about the polonium work, including a description of its manufacturing process. His espionage was not discovered until decades later. In 2007, Russian President Vladimir Putin posthumously awarded Koval the Hero of the Russian Federation, finally revealing his role publicly.
The Courier Network
None of this espionage would have worked without couriers who physically moved information from spies inside the project to Soviet handlers. Harry Gold was the most important, serving as the link between both Fuchs and Greenglass and their Soviet contacts. Morris and Lona Cohen were another crucial pair. Lona Cohen smuggled what has been described as a complete working plan of the first atomic bomb from Los Alamos to a Soviet handler in New York. The Cohens later fled the United States and continued espionage work in England under assumed identities before being caught in 1961.
Why They Did It
The overwhelming pattern among the atomic spies is that they were ideologically motivated volunteers, not mercenaries. The Soviet Union proved especially effective at exploiting the sympathies of Americans and British citizens who were communists or fellow travelers. In most cases, the spies approached Soviet intelligence themselves rather than being recruited. Fuchs, Hall, and Greenglass all held left-wing political views and believed, to varying degrees, that sharing nuclear knowledge with the Soviet Union would create a balance of power and prevent future wars.
The scale of penetration was staggering. Few aspects of the Manhattan Project remained secret from the Soviet Union for long. Given the size of the existing Soviet espionage network in the United States and the number of sympathetic individuals in sensitive positions, intelligence historians have concluded that preventing all penetration of the project was probably impossible.
How the Spies Were Caught
The unraveling began with a secret U.S. Army codebreaking program that started in February 1943, later codenamed VENONA. Cryptologists spent nearly two years learning to break the encryption used in Soviet diplomatic cables. When they finally succeeded, the decoded messages revealed espionage efforts targeting the atomic bomb program. The first public release of translated VENONA messages, in July 1995, included 49 messages about Soviet efforts to steal information on the bomb.
VENONA led investigators to Fuchs, who was arrested in England in 1950 and confessed. His confession led to Harry Gold, which led to David Greenglass, which led to Julius Rosenberg. The program continued to provide intelligence on Soviet espionage until it was shut down in 1980, though its existence remained classified until the mid-1990s.
How Much Did Espionage Matter?
The stolen secrets did not hand the Soviet Union a bomb it could not have built on its own. Soviet physicists were talented and had already begun their own nuclear research. Russian scientists estimated it would take them five years to produce the bomb, and they actually did it in four. General Leslie Groves, who led the Manhattan Project, was far more optimistic about American advantage. He predicted in 1948 that even if the United States had shipped complete blueprints to Russia in 1945, the Soviets would not produce bombs in quantity before 1955, partly because the Americans had cornered known uranium supplies.
The truth fell somewhere between these estimates. Espionage gave Soviet scientists confirmed design data that let them skip the trial-and-error phase of development. The first Soviet bomb was essentially a copy of the American Fat Man design, built using the technical descriptions Fuchs provided. Without espionage, the Soviets would still have built a nuclear weapon, but they would have done so later and with a design they developed independently. The two-year acceleration meant the Soviet Union tested its bomb in 1949 rather than 1951 or later, reshaping the early Cold War and ending the brief American nuclear monopoly far sooner than Washington expected.