Manhattan Project
The most successful Soviet spy was Klaus Fuchs. Fuchs was a member of the British Mission who played an important part at Los Alamos. The 1950 revelation of Fuchs' espionage activities damaged the United States' nuclear cooperation with Britain and Canada. Subsequently, other instances of espionage were uncovered, leading to the arrest of Harry Gold, David Greenglass and Ethel and Julius Rosenberg. Other spies like George Koval remained unknown for decades. People will never know the value of the espionage. One reason was that the Soviet atomic bomb project was held back by a shortage of uranium ore. The consensus is that espionage saved the Soviets one or two years of effort.