Martin and Mitchell defection

In September 1960, two U.S. National Security Agency (NSA) cryptologists, William Hamilton Martin and Bernon F. Mitchell, defected to the Soviet Union. A secret 1963 NSA study said that: "Beyond any doubt, no other event has had, or is likely to have in the future, a greater impact on the Agency's security program." Martin and Mitchell met while serving in the U.S. Navy in Japan in the early 1950s and both joined the NSA on the same day in 1957.

Source: Wikipedia — Martin and Mitchell defection (CC BY-SA 4.0)