Levelling the playing field

Questioning the status of history’s ‘most dangerous’ spy

Clovis Meath Baker

Of all the known Cold War spies, Klaus Fuchs gave the most material help to the Soviet Union. As the key British theoretical nuclear physicist in the US/UK Manhattan project, and then in the UK’s own independent nuclear weapons programme after 1946, Fuchs had an almost unrivalled overview of the whole nuclear cycle, from uranium enrichment to weaponisation. As it turned out, the Russians had several nuclear spies in the US/UK Manhattan project, but these had only partial visibility of the programme as a whole. Fuchs saw the lot. He started early, too, offering his services via communist friends in the UK in 1941. 

At his first meeting with his Soviet case officer, in London, he handed over six pages of notes giving the Russians the biggest secret of all—that a nuclear bomb was a practical possibility: there was a viable process to enrich natural uranium to increase the proportion of fissile U-235; and that the critical mass necessary for an explosion would be small enough to fit in a device which could be delivered by conventional bomber aircraft. 

Soviet military intelligence, the GRU, quickly realised the value of their new source, and ran him using all the classic spycraft of a Le Carré novel—chalk marks on walls, messages written in Men Only and dropped over garden walls, meetings sitting side by side on buses, couriers who did not know what they were carrying acting as cut-outs. To reduce the risk of detection, meetings were infrequent, but over the next eight years, both in England and the US, Fuchs supplied a stream of detailed nuclear intelligence which was of critical help to the Soviet nuclear programme—although Russian scientists have tended to play this down as it detracts from the reputations of their own nuclear pioneers.

Klaus Fuchs, son of a Lutheran pastor, grew up in Germany in the 1920s, and became a brilliant student and committed communist. After Hitler came to power, he escaped to England and completed his PhD at Bristol University, helped on the way by a network of probably communist sympathisers. Interned as an enemy alien for six months in 1940, he was invited to join the nascent British atomic bomb research effort the following year, and within weeks contacted Soviet intelligence. He was one of the British scientists sent to the US for the Manhattan project, and on his return to Britain in 1946 he became head of theoretical physics at the newly-formed Atomic Energy Research Establishment at Harwell. At the same time, supposedly secret from his Harwell colleagues, Fuchs was also advising the project to build the first UK nuclear weapon, based at Fort Halstead in Kent. 

MI5 had studied Fuchs intermittently, and had approved his vetting to join the nuclear programme. But the MI5 vetting officers had no idea what Fuchs was working on beyond a vague description of important secret research with a military application.  Had they known the full gravity of his nuclear work, they might have put more time into their investigation. 

MI5, which had been in a liaison relationship of sorts with the Gestapo before the war, also discounted a German report that Fuchs was a communist on the grounds that the Gestapo might have invented this to discredit him. The MI5 paperwork shows that Roger Hollis, later accused of being a Russian spy himself, consistently argued for giving Fuchs the benefit of the doubt—so providing more food for the conspiracy theorists.

What undid Fuchs—as well as the Cambridge Five, the Rosenbergs, and many others—was signals intelligence (Sigint). The US Army Signal Intelligence Service, under a programme codenamed Venona, was able to decipher a significant proportion of Russian intelligence telegraphic traffic dating from 1943-4. The material was patchy, and the Russians used codenames to disguise their sources, but Venona led FBI and MI5 investigators to identify Fuchs as a possible Soviet agent. 

By 1949, MI5 had decided to put extensive surveillance on Fuchs, to try to catch him red-handed; Venona would not be admissible as evidence in court, and in any case they did not want to blow the Venona secret. But there was nothing for MI5 to find: whether because of a change of ideological heart or nervous of compromise, Fuchs had broken off contact with the Russians. The technical limitations of the time made it hard for MI5. Without modern concealed radio sets, surveillance operators on foot and in cars had to stick closely to their target, who almost certainly knew he was being followed. And MI5 had to assign three people to the local telephone exchange in order to monitor Fuchs’s calls, which of course were innocuous.

In the end the only option for MI5 was to call Fuchs in for interview. After a series of meetings, each carefully planned, recorded, and deconstructed afterwards, Fuchs finally confessed, probably because his interviewer, a former Scotland Yard detective inspector who was not aware of the Venona material, tricked him into it by implying an offer of immunity from prosecution (which was retracted when he realised the full scope of Fuchs’s efforts). 

It was on the basis of this confession that in 1950 Fuchs was convicted of espionage and sentenced to 14 years, the maximum. Released in 1959, he emigrated to East Germany, where he died in 1988—not quite in time to see the collapse of the system for which he had worked so hard.

As a particle physicist well-known for making science accessible to laymen, Professor Frank Close explains the breakthroughs in nuclear theory as well as the engineering and management challenges of what became the Manhattan project with an engaging clarity that a layman can follow. And he brings the same relentlessly scientific approach to his examination of the MI5 investigation and why they missed Fuchs early on. There may be too much detail on this for some, but the chronology of the state’s bureaucratic and legal processes shows a reassuringly measured effort to play by the rules—Fuchs was never physically mistreated, for example, and there was no question that without evidence or a confession he would not have been convicted.

The jury is out on whether Fuchs was “the most dangerous spy in history”, as the title suggests. After Hiroshima, the whole world knew that nuclear weapons were possible. Then as now, any nation with the requisite political will, budget, and scientific and engineering capabilities could build a bomb. The Soviet Union would have got there in the end, but the intelligence Fuchs and others provided helped them get there quicker and avoid engineering dead ends. The first Soviet nuclear test in 1949 (of a device based on a US design) took the West by surprise—the US had estimated it would take the USSR until the mid 1950s. Their brief nuclear monopoly did not enable the Americans to hold back the Soviet takeover of Eastern Europe or communist expansionism elsewhere in the world; and even if the nuclear monopoly had lasted five years longer, it is hard to see how the US could have leveraged this against Stalin. Nuclear weapons may be a nation’s ultimate insurance policy but are simply not that useful in persuading other countries to do what you want.

Fuchs also comes across as morally different from Philby, Burgess and the others. He was not trying to subvert Britain from within, or to blow British agents so that the NKVD could execute them, but to level the nuclear playing field to prevent an American nuclear monopoly. When he started handing over intelligence to the Russians, they were Britain’s allies; and some of what he passed on was his own work. His motivation was ideological, and strong; he was not a seedy alcoholic misfit. He became a British citizen only in 1942, which somehow lessens our sense of betrayal—and it was our own fault for trusting him. 

But Fuchs’s spying strengthened the USSR at a time when Stalin was at his most dangerous for the West, and his exposure and conviction helped create a climate of distrust and paranoia towards his former colleagues, many of them like him refugees from Europe. Although Britain never had a McCarthy-style campaign, the atmosphere changed.

The Manhattan project and Fuchs’s espionage happened over 70 years ago, but there are parallels with today. Nuclear theory is one thing but actually making a viable nuclear weapon is a huge and complex engineering task, difficult, thank goodness, for a country to do in secret—as Iran and North Korea have discovered. Security vetting is an unreliable process, and is especially difficult for people who have lived abroad for significant periods. Detecting spies through physical surveillance alone is really hard. And a big Sigint breakthrough can be the most valuable counter-espionage tool of all.

Professor Close’s thorough and sometimes exciting book is a great window into how the first nuclear weapons were developed, how the Soviets were able to recruit so many agents at the heart of the British state, and how Britain worked in the late 1940s.


Trinity: The Treachery and Pursuit of the Most Dangerous Spy in History
By Frank Close
Allen Lane, 528pp, £25

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