Claim: Kim Philby, part of the Cambridge Five, was an effective American spy while he was serving in the British counterintelligence service.
Evidence: Harold Adrian Russell "Kim" Philby (1 January 1912 – 11 May 1988) was a British intelligence officer and a double agent for the Soviet Union. Of the five, Philby is believed to have been most successful in providing secret information to the Soviets.