1. Julius and Ethel Rosenberg

Ethel and Julius Rosenberg. (Credit: Heritage Images/Getty Images)

Married in 1939, New York City residents Julius and Ethel Rosenberg were devoted communists who allegedly headed a spy ring that passed military secrets to the Soviets. The scheme got underway sometime after 1940, when Julius became a civilian engineer with the U.S. Army Signal Corps. He was dismissed in 1945 once the military learned of his communist sympathies, but not before recruiting Ethel’s brother, an Army machinist working on the Manhattan Project, to turn over handwritten notes and sketches pertaining to the atomic bomb. Meanwhile, other Rosenberg recruits purportedly delivered thousands of pages of documents detailing new radar and aircraft technologies. At trial following their 1950 arrest, Ethel’s brother testified against them, and a judge sentenced them to death, declaring their crime “worse than murder.” President Dwight D. Eisenhower then sealed their fate by denying a petition for executive clemency. The two were sent to the electric chair at New York State’s Sing Sing prison on June 19, 1953, marking the first time American civilians had ever been executed for espionage. Although worldwide protests erupted over the Rosenbergs’ treatment, with many people feeling they had fallen victim to McCarthy-era red baiting, the post-Soviet release of decrypted KGB messages proved that Julius had in fact been a spy. The evidence against Ethel is less ironclad, and her guilt remains in dispute.

2. Klaus Fuchs

Police photograph of Physicist Klaus Fuchs. (Credit: Public Domain)

Following Adolf Hitler’s rise to power in 1933, Klaus Fuchs fled his native Germany for the United Kingdom, where he received a doctorate in physics and eventually became a citizen. During World War II he was invited to join Britain’s clandestine atomic bomb development program, despite his known communist leanings, and from there was sent to the United States to take part in the Manhattan Project. Upon returning to the U.K., Fuchs secured a prestigious post at a nuclear energy research center. In 1950, however, he was apprehended after U.S. agents discovered that for years he had been handing nuclear secrets to the Soviets, who by now had their own atomic bomb. Fuchs confessed, telling the authorities that he “had complete confidence in Russian policy” and that “the Western Allies deliberately allowed Russia and Germany to fight each other to the death.” Though Fuchs claimed not to know his American contact’s true name, the FBI quickly traced a trail back to the Rosenberg spy ring, resulting in the arrest of the Rosenbergs and several co-conspirators. Compared to the Rosenbergs, Fuchs got off easy. After nine years in British prison, he immigrated to East Germany, where he continued working as a nuclear physicist until his retirement in 1979. A winner of the Karl Marx Medal, East Germany’s highest civilian honor, Fuchs died in 1988 at age 76.

3. Ray Mawby

The London Houses of Parliament. (Credit: Public Domain)

Ray Mawby, a one-time electrician, served from 1955 to 1983 in the House of Commons, where he championed so-called traditional British values (he campaigned, for example, against the legalization of homosexuality). For Conservative Party members like him, hatred of communism was practically a prerequisite. Yet in 2012, a dozen years after his death, a BBC reporter unearthed a file showing that Mawby had been a mole for Czechoslovakia, then part of the Soviet bloc. Hundreds of pages of documents revealed that Mawby, who was given the codename Laval, began secretly handing over intelligence not long after Czech agents first approached him at a November 1960 cocktail party. Lacking access to classified information, Mawby supplied them instead with political gossip, such as the existence of a confidential investigation into a Conservative Party colleague. More insidiously, he also apparently provided floor plans of the prime minister’s parliamentary office, as well as details about the prime minister’s security team. For each helpful tidbit, Mawby received £100, which, his handlers implied, went toward his drinking and gambling habits. In later years, they upped the total to £400 per year. Though Mawby at one point met several times a month with his handlers, their collaboration appears to have ended in 1971. Remarkably, some Labour Party politicians are also known to have been in cahoots with the Czechs.

4. The Cambridge Five

Two members of the Cambridge Five. (Credit: Hulton Archive/Getty Images)

Incredulous that a Conservative member of Parliament could be a communist spy, the British authorities were likewise thrown off by the elite educations and upper-class backgrounds of the so-called Cambridge Five, who were recruited into the Soviet sphere around the time they attended the University of Cambridge in the 1930s. Within a decade or so of graduation, Donald Maclean, Guy Burgess, Kim Philby, Anthony Blunt and John Cairncross had all worked their way up to important intelligence posts, which they used to pass an array of secrets to the Soviets. For example, thanks to these double agents, who were reportedly motivated by ideology, not money, the Soviet Union learned about an Allied plan to send anti-communist insurgents into Albania, as well as Allied military strategy during the Korean War. Upon discovering that the authorities were closing in, Philby, who ironically headed the anti-Soviet section of MI6 (the British equivalent of the CIA), tipped off Maclean and Burgess, prompting them to defect to Moscow in 1951. Philby joined them there in 1963, whereas Cairncross ended up in Italy and France. Blunt, meanwhile, confessed in exchange for immunity from prosecution and was allowed to stay in Britain. None of the five ever faced espionage charges.

5. Aldrich Ames

Former senior Central Intelligence Agency office Aldrich Hazen Ames. (Credit: LUKE FRAZZA/AFP/Getty Images)

The son of a CIA analyst, Wisconsin-born Aldrich Ames wasted no time in joining the agency himself, starting there in high school as a part-time clerical worker and later becoming a full-fledged spy. Posted to such places as Turkey and Mexico, Ames spent much of his three-decade-long career attempting to coax Soviet officials into the CIA’s service. Despite an obvious drinking problem and poor performance reviews, he advanced to become head of the counterintelligence branch of the CIA’s Soviet division. In 1985, however, while going through a financially disastrous divorce, Ames walked into the Soviet embassy in Washington, D.C., and offered to trade secrets for money. Paid some $2.7 million over the next nine years, he in return left classified documents at prearranged drop sites for the KGB to pick up later. He moreover disclosed the identities of virtually every secret agent working for the Americans within the Soviet Union, at least 10 of whom were subsequently executed. “[They] died because this warped, murdering traitor wanted a bigger house and a Jaguar,” the CIA’s director said later. Though U.S. officials had suspected the existence of a mole for quite some time, Ames avoided arrest until 1994, when the FBI finally closed in after uncovering incriminating evidence in his trash and on his computer. He is currently serving a life sentence at a federal prison in Pennsylvania.

6. Adolf Tolkachev

The previous five examples notwithstanding, not every traitorous Cold War spy supported the communist cause. In early 1977, for instance, Soviet electronics engineer Adolf Tolkachev began dropping notes into the cars of U.S. diplomats, asking to meet with an American official. The CIA originally ignored him, worried that it would fall into a KGB trap. But Tolkachev, who worked at a military aviation institute in Moscow, persisted and eventually gained the CIA’s trust. From 1979 to 1985, he regularly stuffed classified documents into his coat in order to photograph them at home with a CIA-supplied camera. His CIA handlers would then intermittently pick up this film, along with handwritten messages, after taking great care to avoid KGB surveillance. From Tolkachev, the CIA learned that U.S. cruise missiles and bomber planes could fly under Soviet radar. It also gained great knowledge of new Soviet weapon systems, thus saving the U.S. military an estimated $2 billion in research and manufacturing costs. For this spy work, the CIA paid Tolkachev more than $1 million—the majority of which was held in escrow pending his planned defection—and supplied Led Zeppelin, The Beatles and other Western rock albums for his son. Yet he appears to have been motivated more by revenge than money, telling his CIA handlers about the murder of his wife’s mother and the imprisonment of her father during Joseph Stalin’s purges of the 1930s. (Tolkachev was furthermore upset by the government’s treatment of contemporary dissidents he admired.) The collaboration came to an abrupt end in 1985, when it’s believed that former CIA agent Edward Lee Howard, and possibly Aldrich Ames as well, told the Soviets about Tolkachev’s activities. He was executed the following year.