Nuclear espionage
Klaus Fuchs – A physicist who had escaped Germany and who worked with the British at Los Alamos during the Manhattan Project. He was eventually discovered. He confessed, and was sentenced to jail in Britain. He was later released, and he moved  to East Germany. Because of his close connection to many parts of the project, and his deep technical knowledge, he is considered to have been the most valuable of the "Atomic Spies" in terms of the information he gave to the Soviet Union about the American fission bomb program. He also gave early information about the American hydrogen bomb program but since he was not there at the time that the successful Teller-Ulam design was discovered, his information on this is not thought to have been of much value.