Klaus Fuchs was just one of the many eccentrics chosen to work on the Manhattan Project. People remembered him as being serious, quiet, and earnest. He was also a spy — whose eventual capture lead to both the Red Scare and the arrest of the Rosenbergs.
When choosing scientists for the Manhattan Project, the organizers of the project were in a bind. They were up against an enemy who had captured supplies of many of the materials necessary to make a bomb, that had been a Mecca for brilliant physicists before the war, and that still retained a great deal of talent.
They were making a huge investment of time and money during a period where every dollar and second counted. They needed the best talent. But they also needed to think about security. “The best talent” was often paired with idiosyncratic politics and personalities.
For example, Richard Feynman, though not disloyal, had a disregard for security protocol, and at times had a love of breaking into places just to show that he could. This was a headache, and a worry, for the military. In fact, it had got around the facility that Richard Feynman had sat up one night drinking with another scientist, and had loudly announced that clearly, he was the person in the room most likely to be a spy.
Feynman, for once, was wrong. It was the other scientist who was the spy. Klaus Fuchs was a serious and quiet man, who had studied graduate level physics at Edinburgh University.
He was a German who had even been placed briefly in an internment camp by the British government — but he wasn’t likely to have been a Nazi spy. He had fled Germany to get away from the Nazis, and his stay in the camp was cut short when his teachers went en masse to the British government and pleaded for his release.
On the strength of their say-so, and on his stellar work in tube alloys as part of the war effort, he was assigned by the British to be one of their scientists working at Los Alamos on the Manhattan Project. The British hadn’t considered one thing when they made this decision.
While there was no chance that he would spy for the Nazis, Fuchs had fled to Germany because he had been a communist. Obviously there’s a difference in being a communist and spying on your country for a communist country. (We all learned that the hard way.) But Fuchs had been a spy even when he was working on alloys.
His statement doesn’t entirely hold up, as he continued to spy for the Soviet Union well after the war ended. It was only in 1949 that suspicion fell on him. Britain and the US launched the VENONA Project, a codebreaking operation.
The Russian codes that had been used by agents during the war and afterwards were decrypted with special keys that were used only one time each. When they were used that way, the codes were considered unbreakable.
VENONA only managed to untangle the messages because the Soviets had the same problem as the Manhattan Project supervisors. They had to balance a wartime need against the interests of security, and so occasionally re-used keys, or reproduced keys, in order to keep up with the demand
At first when VENONA officials found Fuchs‘ reports, they hoped that someone in his office had stolen them. As more evidence piled up, they had to admit that that wasn’t the case.
They arrested Fuchs, who, facing the consequence of turning over nuclear secrets, was eager to inform on his fellow spies in order to make a deal.
He gave up his courier, Harry Gold, who, when he was arrested, gave up David Greenglass, another spy at Los Alamos and brother to Ethel Rosenburg. It was Greenglass who turned in Julius and Ethel Rosenberg.
They protested their innocence, and it was their trial, as well as the realization that the most guarded secret facility of World War II had had multiple spies working in it, that started the Red Scare. Fuchs’ own trial lasted all of 90 minutes, after which he was sentenced to fourteen years imprisonment.
After serving just under ten, he was sent to East Germany, where he resumed his career — and some say he was instrumental in China developing the atom bomb. He died in 1988.
Via UMKC, Atomic Archive, PBS, Before the Fallout, iO9 Gizmodo and About.com.