Klaus Fuchs

If one spy shaped the nuclear balance, it was Fuchs. A German-born physicist, he worked in the Theoretical Division at Los Alamos under Hans Bethe and Edward Teller and passed atomic secrets to the Soviet Union. He was not caught by a slip. Decrypted Soviet cables from the Venona project exposed him in 1949, he was arrested in January 1950 and admitted spying, and he received 14 years, serving nine before his release in June 1959. Estimates credit his material with letting Moscow test its bomb one to two years sooner. His arrest also pulled the thread that led investigators to Harry Gold and on to the Rosenberg network.