Klaus Fuchs
German-born Manhattan Project physicist and committed communist who handed the Soviet Union the plutonium bomb design, ending the American nuclear monopoly early.
He was one of the most valuable spies of the twentieth century, and he did it out of conviction rather than money. Klaus Fuchs was a German-born physicist, a refugee from Hitler, and a committed communist who spent the war inside the Manhattan Project. From that seat he handed the Soviet Union the design of the plutonium bomb, and in doing so helped erase the American nuclear monopoly years ahead of schedule.
From refugee to insider
Fuchs was born near Frankfurt in 1911, the son of a Lutheran pastor. He joined the German Communist Party as a student and fled the country in 1933, weeks after the Nazis took power, eventually reaching Britain. When war came the British interned him as an enemy alien and shipped him to a camp in Canada, then thought better of it: a physicist of his caliber was too useful to waste. Released, he went to work for Rudolf Peierls in Birmingham on Britain's bomb project, was naturalized as a British citizen in 1942, and in 1943 crossed the Atlantic with the British delegation to the Manhattan Project.
By August 1944 he was at Los Alamos, in the Theoretical Physics Division under Hans Bethe, working on the hardest problem in the building: the implosion mathematics for the plutonium bomb. Almost no one was better placed to know how the weapon actually worked. He stood in the desert and watched the Trinity test in July 1945.
What he gave away
Fuchs had begun passing secrets almost as soon as he had them, first through a courier in Britain and then, in the United States, through a Philadelphia chemist named Harry Gold. What he delivered was not gossip but the blueprint: the implosion principle, the solid plutonium core, the initiator that triggered it, the expected yield, even where and when Trinity would be fired. Later he handed over early work on the hydrogen 'Super.' Soviet physicists, badly outnumbered and driven under Beria's whip, used it to check and shortcut their own effort. When the USSR detonated its first bomb in August 1949, it was a near-copy of Fat Man, and it arrived so early that Washington at first refused to believe it.
Exposure
Decoded Soviet cables, the Venona intercepts, eventually pointed at a scientist inside the project. Under questioning by MI5's William Skardon, Fuchs confessed in January 1950, not because he was cornered by hard proof but, by his own account, because he had come to doubt the cause. He was sentenced to fourteen years, served nine, and on release in 1959 moved to communist East Germany, where he lived out his life as a respected physicist and died in 1988.
His confession set off the chain that led to Harry Gold, David Greenglass, and the Rosenbergs, and it poured fuel on the American spy panic that became McCarthyism. But the deeper lesson outlived all of them. A single trusted man with a clear conscience had shown that the most closely guarded secret on earth could not be kept behind a fence. Physics, once discovered, does not stay put.