Security around the Manhattan Project was quite intense. No doubt as a result of this Truman assumed that the Soviets had no inkling about the bomb. Perhaps he was therefore surprised when Stalin had little reaction when Truman at the Potsdam Conference told him that the US had developed an atomic bomb. Notwithstanding the intense security, the Soviets were kept quite informed about the steps the United States was taking to develop the Atomic Bomb, thanks to various spies and traitors:
1. Klaus Fuchs-A German anti-Nazi refugee to Britain, and a physicist, Fuchs worked at Los Alamos among the elite of the physicists. A convinced Communist, Fuchs had his information sent on the Soviets, courtesy of Harry Gold.
2. Harry Gold-An American laboratory assistant, Gold had been passing information to the Soviets since 1934. Here is his report on his first meeting with Fuchs:
He (Fuchs) obviously worked with our people before and he is fully aware of what he is doing… He is a mathematical physicist… most likely a very brilliant man to have such a position at his age (he looks about 30). We took a long walk after dinner… He is a member of a British mission to the U.S. working under the direct control of the U.S. Army… The work involves mainly separating the isotopes… and is being done thusly: The electronic method has been developed at Berkeley, California, and is being carried out at a place known only as Camp Y… Simultaneously, the diffusion method is being tried here in the East… Should the diffusion method prove successful, it will be used as a preliminary step in the separation, with the final work being done by the electronic method. They hope to have the electronic method ready early in 1945 and the diffusion method in July 1945, but (Fuchs) says the latter estimate is optimistic. (Fuchs) says there is much being withheld from the British. Even Niels Bohr, who is now in the country incognito as Nicholas Baker, has not been told everything.
3. David Greenglass-A machinist at Los Alamos, he passed information on to his sister Ethel Rosenberg and her husband Julius Rosenberg who passed it on to the Soviets.
4. Theodore Hall-An American physicist working at Los Alamos. Perhaps the most important of the atomic spies, beginning in October 1944 he passed on detailed information to the Soviets about the construction of the atomic bomb code-named “Fat Boy”.