Klaus Fuchs, according to a 1951 report by the US Congress, “influenced the safety of more people and accomplished greater damage than any other spy not only in the history of the United States but in the history of nations”. That judgement seems unarguable: from 1941 to 1949 Fuchs was at the centre of the British and American nuclear programmes and he provided the Soviets with every secret that came into his hands, from the fruits of Britain’s early research into a fission device using uranium’s unstable isotope U235, to the discovery of the explosive potential of plutonium, to the theory behind a much more powerful hydrogen bomb using nuclear fusion. It was largely because of Fuchs that the Soviet Union tested its own atomic bomb in 1949, to the astonishment of the US and Britain.

Although it appears to endorse this judgement in its subtitle, Frank Close’s Trinity presents a considerably more nuanced picture of the “most dangerous spy in history”. Close does not play down the impact of Fuchs’s espionage, but provides three arguments in retrospective mitigation for his crimes. The first is Fuchs’s personal history.

He was born in 1911 near Frankfurt. His father was a strait-laced Lutheran who became a Quaker, and his mother suffered from such severe depression that she took her own life by drinking hydrochloric acid when Fuchs was 19 – one of three suicides among women in his immediate family. Fuchs joined the Social Democratic Party but, as German politics became more polarised in the late 1920s, he followed his sister into the German Communist Party. This made Fuchs a target for Hitler’s Brownshirts, a gang of whom knocked out his front teeth in an attempted lynching. The dental plate and false teeth he wore for the rest of his life were a constant reminder of the consequences of political action. In a poignant detail, when he was finally run to ground by MI5 in 1949, Fuchs’s dental plate broke and he had to cut short an interrogation session in order to see a dentist.

The second argument in Fuchs’s defence is his apparent motivation for betraying his adopted country. The UK provided a safe haven for Fuchs in 1933, although with the Germans threatening to invade in 1940, British authorities interned him in Quebec – a stupid and counterproductive policy, considering that most of the 2,000 “enemy aliens” sent to Canada were Jewish refugees from Nazi Germany. (It was also in Canada that Fuchs met the communist Hans Kahle, who may have been acting as a talent-spotter for Soviet intelligence and who certainly introduced Fuchs to a group of Russians in Hampstead that included Fuchs’s Soviet military intelligence recruiter.) Despite his debt to Britain, Fuchs claimed to believe that atomic secrets were too important to be monopolised: awaiting trial in Brixton Prison in 1950, he was overheard telling his friend Rudolf Peierls – the German-born physicist who had recruited Fuchs into Britain’s secret atomic programme – that “knowledge of atomic research should not be the private property of any one country” but should be “shared with the world for the benefit of mankind”.

Close appears to accept this high-minded line at face value, although elsewhere he also endorses the view that Fuchs was a committed, lifelong communist. But Peierls was a physicist of particular integrity who, at the very outset of his atomic research, saw that mankind could only survive in the nuclear age if it pursued a doctrine of deterrence. Fuchs’s explanation seems tailor-made to appeal to Peierls’s enlightened world-view.

The third and most powerful argument in Fuchs’s favour is that the Soviets were not the only beneficiaries of his licit and illicit knowledge. After the 1946 McMahon Act froze out Allied states, including Britain, from American nuclear research, Fuchs became crucial to the British civil and military nuclear programme. But even before becoming head of the Theoretical Physics Division at the newly created Atomic Energy Research Establishment at Harwell, Fuchs had, it seems, supplied Britain with American nuclear secrets, which helped ensure that Britain became the world’s third nuclear power in 1952. More fundamentally, whereas the “Cambridge Five” were largely unproductive in their day jobs and distinguished only by their treachery, Fuchs made significant personal contributions to nuclear physics, and even greater contributions when working with others such as the celebrated mathematician John von Neumann. Indeed, Close suggests that Fuchs deserves to be credited as the grandfather of the H-bomb, as his insight that a successful explosion required “radiation implosion” was fundamental to the American, Russian and British designs.

The second half of Close’s book narrates the gradual tightening of the net around Fuchs, as British and American intelligence belatedly realised he was a spy and tried to ensure he was neutralised without causing a scandal. Close recites a litany of missed opportunities, bureaucratic bungles and the odd bit of self-serving skulduggery. However, given the scale and significance of Fuchs’s achievements, it is difficult to avoid the conclusion that the failure to identify him as a Soviet agent at an earlier stage was a blessing in disguise.

Information about his communist sympathies was at various stages unavailable, deliberately withheld or misinterpreted by both British and American agencies. But had Fuchs been excluded from the Manhattan Project on national security grounds in 1943, the Soviets would not have been the only losers.

At its heart, the Fuchs story is about knowledge – how it is created, how it is used and misused, and how it is protected from friends and enemies. Among several ironies is that necessary but extreme secretiveness ultimately compromised both British and American security. One explanation for the failure to investigate Fuchs properly was that the officials responsible for security vetting did not know the importance of his role. Thanks to Fuchs, therefore, Joseph Stalin knew considerably more about what was going on at Los Alamos than almost everyone in the American and British governments: compartmentalising that knowledge hindered the Allies but failed to protect the nuclear crown jewels. Similarly, the Americans’ deciphered intercepts of Soviet diplomatic communications – which yielded the clues that led to Fuchs – were almost as tightly held, causing numerous difficulties in identifying Fuchs as a spy, persuading him to confess, and ensuring his conviction.

Fuchs’s story has been told many times but, with the benefit of declassified intelligence files in Britain, America and Russia, Trinity is by far the most comprehensive account so far, and Close’s diligence in the archives is deeply impressive. It is just a shame that his book is not more focused and concise. Bugged conversations are reproduced at inordinate length and there is a wealth of quotidian detail from MI5’s surveillance of Fuchs which, Close acknowledges, was almost completely unrevealing. There is much repetition – sometimes the same material appears twice on consecutive pages – and many reminders of things we have already been told, or previews of things we will be told in more detail later.

It is difficult to avoid suspecting that Close, formerly professor of theoretical physics at Oxford, started writing one book – about Fuchs and his relationship with Peierls, who is the subject of both the prologue and the opening chapter, and who taught Close in the 1960s – and ended up writing another, focusing on Fuchs’s career in espionage. As a result, we know what Fuchs did and how, but his personality and motivation remain largely undiscovered. The man who did more than anyone to ensure a balance of terror between East and West, and an era of Mutual Assured Destruction, remains – like the Soviet Union, which he served clandestinely for eight years – a riddle, wrapped in a mystery, inside an enigma. l

Andrew Glazzard is a senior research fellow at the Royal United Services Institute

Trinity: The Treachery and Pursuit of the Most Dangerous Spy in History

Frank Close

Allen Lane, 528pp, £25