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Klaus Fuchs: the scientist who became the Soviet super spy

Communist scientist Klaus Fuchs lived a double life: an esteemed member of the team working on the British and American nuclear programmes on one side, while on the other he dutifully handed over the West's research to the Soviets, which helped them build the bomb. Michael Goodman delves into the hidden world of Cold War espionage and Fuchs's spy career...

Published: November 29, 2021 at 5:47 am
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“It was a foggy evening when I emerged from the Green Wood underground station. The Nag's Head was across the street and I opened a newspaper, pretending to be waiting for a bus. Everything appeared quiet and clear. At ten minutes to eight a man came around the corner and went into the pub. He was tall and thin and held his head high as he walked. I knew it was Fuchs even before he entered the Nag's Head. I waited a few minutes to make sure no one was following him and then walked up to the pub myself…”


For the best part of a decade, Dr Klaus Emil Julius Fuchs provided a steady stream of information to the Soviet Union. The meeting in the Nag's Head pub in 1947 would be the first with Alexander Feklisov, his new KGB handler, since returning from the wartime Manhattan Project, the secret US effort to build an atomic bomb.

There are often allegations made about how spies have changed the course of history. In the case of Klaus Fuchs, this is most certainly true: even by conservative estimates, it is claimed his information saved the Soviet Union two years in constructing their first atomic bomb.

Fuchs was, in many respects, the typical scientist. Markus Wolf – head of East Germany’s secret police force, the Stasi – gave his first impression of Fuchs: “He was a cartoonist’s notion of the brilliant scientist, with a high forehead and rimless glasses out of which watchful eyes stared thoughtfully… these eyes came to life when Fuchs began to talk about theoretical physics. He had a boyish enthusiasm for the subject.”

From scientist to spy

Fuchs first arrived in the United Kingdom in 1933, fleeing his native Germany and certain Nazi prosecution. He was not a Jew – as many accounts have claimed – but was a communist with Quaker heritage. As an active member of the communist underground movement, his name became known to the Gestapo and he was, according to one of his biographers, “a wanted political criminal”.

His political allegiance was made known to British security authorities upon his arrival, yet no action was taken. He completed his doctorate at Bristol University and, in 1937, moved to Edinburgh University to undertake post-doctoral work, but the advent of war saw his career alter, radically. Following a short spell in Canadian internment, he returned to England and, with Britain’s increased need for competent scientists, the young German became a naturalised British citizen and was provided with security clearances so he could set to work on secret governmental work.

When and why did Klaus Fuchs become a spy?

As a student at Leipzig University, Klaus Fuchs, as many undergraduates do, joined a student political organisation. A Social Democrat, Fuchs abhorred Nazism and marched against the activities of Hitler’s stormtroopers. Following a move to Kiel University, he continued his involvement in politics, becoming chairman of a similar group.   Amongst the members was a sizeable communist contingent. In 1932, and with the Social Democrats supporting Paul von Hindenburg as the next German president, Fuchs split from the party. The German Communist Party instead supported a working-class coalition with the socialists in an attempt to dislodge Hindenburg and Hitler.   In Fuchs's mind, the communists were the only party standing up in opposition to Nazism. This, incidentally, was a common rationale for the recruitment of Soviet spies in the 1930s. In addition to their resistance to Hitler, Fuchs was taken by the utopian idealism offered by communism, seeing it as the great hope for the world.   He continued to antagonise Nazi opponents, attending protests and marches, but his position soon became untenable. Following the Reichstag fire, with the communists being blamed, Fuchs decided it was no longer sensible to remain an open communist in Germany, and he ended up in the UK.

"An evil negligence"   Communism may have had a wide appeal at the time, but there was a quantum leap between supporting a political movement and deciding to provide secret information to a foreign government. The events of the second-half of the 1930s, in particular the Soviet support for the Spanish republican movement, convinced Fuchs of his allegiance. He made contact with a representative of the German Communist Party in the UK, who, unbeknownst to Fuchs, was in fact a member of Soviet military intelligence.   “I could not see why it was in the West’s interest not to share the bomb with Moscow… it was abhorrent to me that one side should be able to threaten the other with such great force… I never thought that I was doing something culpable by passing the secrets to Moscow. It would have seemed an evil negligence for me not to have done it,” he later declared.   Fuchs was motivated by political conviction, not money. In discussions with the Stasi just before his death, Fuchs also clarified that this decision to aid the Soviet Union had been prompted by the fact that the Germans were building a bomb and he believed the Russians should know.    In meeting his various handlers, Fuchs furnished both handwritten and typed copies of reports. As the Soviet nuclear weapons programme increased, he would be asked specific questions to answer, but in the early stages he provided much of what he could get his hands on. This included not only his own work, but other people’s too.   Towards the end of his espionage career, Fuchs had serious doubts over the utility of his actions. He had become dismayed with communism and had made the decision to stop providing information. He never wrote any memoirs, never defended his actions, and never spoke publicly about them.

The discovery in the late 1930s that the atom could be split through the process of nuclear fission was quickly followed by a realisation that this could have military implications. Fuchs was employed on something known as 'Tube Alloys', the British codename for the atomic bomb. His first work was to consider intelligence reports on the embryonic German atomic bomb programme, but quickly he became embroiled in research for Britain’s own nuclear weapon.

In August 1943, Prime Minister Winston Churchill and US President Franklin D Roosevelt signed an agreement in Quebec, cementing the Anglo-American atomic partnership. As a consequence, all developmental activity was moved to the US, which included the relocation of several British scientists to Los Alamos, New Mexico (the home of the Manhattan Project bomb programme led by Robert Oppenheimer). Among them was Fuchs.

Listen: Writer and espionage historian Trevor Barnes discusses the thrilling 1960s MI5 investigation into the infamous Portland Spy Ring, one of the most dangerous KGB espionage networks ever to operate in the UK, on this episode of the HistoryExtra podcast:

Schizophrenic state of mind

By this stage, the dichotomy of Fuchs’s persona was already taking shape – for while his reputation as a scientist expanded exponentially, his espionage role was also beginning to blossom. He had first provided information as early as 1941.

In his later confession to MI5, he described his schizophrenic state of mind: “I used my Marxist philosophy to establish in my mind two separate compartments. One compartment in which I allowed myself to make friendships, to have personal relations, to help people and to be in all personal ways the kind of man I had been before with my friends in or near the Communist Party. I could be free and easy and happy with other people without fear of disclosing myself because I knew that the other compartment would step in if I approached the danger point. I could forget the other compartment and still rely on it.”

In December 1943, with his relocation to the US, Fuchs was given a new Soviet handler. To give an idea of just how prolific he had been in passing information to the Soviet Union, up until then he had provided more than 570 sheets of valuable material. In mid-1944, he moved to Los Alamos. Despite working in the theoretical division, Fuchs and the British Mission were far less compartmentalised than their American counterparts and, as a consequence, he was able to access a wide breadth and depth of information.

With the end of the war, Fuchs, at the insistence of the Los Alamos director, stayed on for an extra year. When he did return to England in 1946, he assumed the position of head of the theoretical physics division at the Atomic Energy Research Establishment, Harwell. While not working on bomb physics per se, Fuchs was a regular visitor and lecturer at Fort Halstead, the home of Britain’s nuclear weapons programme in the late 1940s.

There remained some question marks over his security, but whenever such points were raised it was repeatedly decreed that his usefulness outweighed any potential security risk

All the while, he continued to supply a steady stream of reports to the Russians, beginning with his first meeting in 1947 with Feklisov at the Nag's Head pub. Despite Fuchs’ exalted position, there had remained some question marks over his security, but whenever such points were raised it was repeatedly decreed that his usefulness outweighed any potential security risk.

During the latter stages of the war and the start of the Cold War, Anglo-American code-breakers had worked on cracking Soviet ciphers. This project, codenamed Venona, lasted for a number of years until the Soviets changed their cipher system in the late 1940s. Venona provided a wealth of information on the activities of numerous Soviet spies. The problem was that it only referred to them by their codenames, and a sizeable portion were not identified.

The identity of 'Charles'

Amongst these was a spy known only as 'Charles'. Examination of what details were known about Charles led investigators to conclude that he could only be Fuchs – the facts fitted too precisely for it to be anyone else. The problem was that this information could not be revealed in court because the code-breaking effort was so secret. As such it was necessary to convince Fuchs to confess to his crimes.

In late 1949, Fuchs had gone to see the security officer at Harwell to discuss his father’s appointment to a university chair in East Germany. Henry Arnold, the security officer, had become friendly with Fuchs and they had spent many hours playing chess together. Fuchs was extremely concerned that his father’s appointment would mean he would have to leave Harwell and secret work.

By this time. Arnold knew of the incriminating evidence provided by Venona, confirming Fuchs to be a spy. He played on Fuchs’s insecurities about his father and invited William Skardon, an MI5 investigator, to speak to Fuchs about the situation, which he readily agreed to.

Fuchs talked of having two identities – scientist and spy – and all of a sudden the dividing line between the two was blurring beyond recognition

On 21 December 1949, Skardon met Fuchs for the first time. Sitting in his Harwell office, Fuchs described his background in Germany and his political views. While discussing his time in the US, Skardon suddenly asked whether Fuchs had been in touch with Soviet intelligence. Although Fuchs denied the allegation, he was thrown off balance. Throughout his espionage career he talked of having two identities – scientist and spy – and all of a sudden the dividing line between the two was blurring beyond recognition.

Fuchs and Skardon met again nine days later. Fuchs, who had just turned 38, continued to talk about his past. In a skilful move, Skardon managed to persuade him that whatever information he had passed had been a mistake, and that it would be far better for him to admit it so that he could resume his work. Fuchs was lost in thought over the New Year period but had, it would seem, made up his mind.

On 22 January 1950, he rang Arnold and said he would like to meet. Two days later, a meeting was arranged with Skardon and Fuchs confessed to being a Soviet spy.

Fuchs had initially thought that he might deny everything and leave Harwell to start an academic career. Yet in his confession Fuchs stated why he had felt it necessary to come clean: “It then became clear to me that in leaving Harwell in those circumstances I would do two things. I would deal a grave blow to Harwell, to all the work which I had loved and, furthermore, that I would leave suspicions against people whom I loved, who were my friends and who I believed I was their friend. I had to face the fact that it had been possible for me in one half of my mind to be friendly with people, be close friends and at the same time to deceive them to endanger them.”

During the course of several conversations, Fuchs admitted his role in passing information to the Soviet Union, and in March 1950 was convicted at the Old Bailey to 14 years’ imprisonment.

Cold War espionage: the spies working for the Soviets

Kim Philby Kim Philby is perhaps the best known and the most destructive of the Cambridge Five – a group of Cambridge University students who became the Soviet Union’s best spies. Recruited in the 1930s to the communist cause, Philby worked for MI6, eventually becoming head of the Soviet counterespionage directorate and liaison in the US. After several successful attempts at diverting attention away from himself, Philby defected in 1963.   Donald MacLean Son of a Liberal cabinet minister, Maclean was another of the Cambridge Five, enjoying a successful and promising career in the Foreign Office. He was identified through the decryption of Venona traffic. Before he could be arrested, he managed to escape abroad in 1951 and lived out his life in the Soviet Union.   George Blake A member of British intelligence who spied for the Russians, Blake was converted to the communist ideology whilst a prisoner of war during the Korean War. Following his release, he joined MI6 and gave away a host of information, including details of the secret eavesdropping tunnel beneath Berlin. Blake was arrested and sentenced to 42 years’ imprisonment, but was broken out of jail five years later in 1966.    Allan Nunn May Revealed in 1946 following the defection of Soviet cipher clerk Igor Gouzenko, Nunn May was a physicist who had worked on the Manhattan Project. Following the war, he became a lecturer at King’s College London, where he was arrested outside a class. He provided the Russians with important scientific information, including a sample of fissionable uranium.   The Rosenbergs Perhaps the most infamous of the US spies who gave information to the Russians, Julius and Ethel Rosenberg continue to attract interest because they were executed for treason in 1953. Not spies in the traditional sense, they were couriers – passing information from agents to their KGB masters. Most of their work was to do with nuclear secrets, though they also provided the Russians with a proximity fuse, a useful invention used to shoot down the spyplane of US pilot Gary Powers over Russia in 1960.

It was only the end of the Cold War and the opening of the Russian archives that we are now in a position to assess the role Fuchs’s espionage had on the Soviet programme, not to mention the nuclear arms race in general. Yuli Khariton, the Soviet chief nuclear weapons designer, has commented how “the design of the first Soviet atomic bomb was based on a rather detailed diagram and description of the first American bomb, which the Soviet Union obtained through the efforts of Klaus Fuchs and Soviet intelligence”.

Whilst Soviet scientists were developing their own indigenous atomic bomb, it was decided that in the first instance it made sense to copy the designs provided by Fuchs. It was known to work. This may sound rather simplistic now, but until the successful test of the world’s first atomic device in July 1945, no-one could be entirely sure that it would ever go bang.

Whilst Soviet scientists were developing their own indigenous atomic bomb, it was decided that in the first instance it made sense to copy the designs provided by Fuchs

Fuchs, although an excellent scientist, was politically rather naïve. His motives for providing information to the Soviets were not only that he believed, at least initially, in their Marxist cause, but that he did not want the US to have a monopoly over atomic weapons.

This latter belief may appear commendable 60 years later; it was not at that time. Yet by also aiding the British in much the same way as he had helped the Russians, Fuchs did ensure that the US monopoly did not last long. Fuchs’s biggest regret after his arrest was not over the enormity of his crimes, rather that the British government had decided to rescind his citizenship.

He returned to East Germany in 1959 a repentant man who, until the year before his death, refused to comment on what he had done.

Dr Michael Goodman is a professor of intelligence and international affairs and head of the department of war studies at King’s College London. He is also the author of Spying on the Nuclear Bear: Anglo-American Intelligence and the Soviet Bomb (Stanford University Press, 2008)


This article was first published in the February 2007 issue of BBC History Magazine


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