In February 1943, the U.S. Army's Signal Intelligence Service began a secret program to decrypt Soviet diplomatic and intelligence communications. Codenamed Venona, the project exploited a critical Soviet error: overworked cipher clerks had sometimes reused pages from their one-time pads, which theoretically should never be reused.
Over the next 37 years, Venona analysts decrypted approximately 3,000 messages out of hundreds of thousands intercepted. The decrypts revealed an extensive Soviet espionage network operating in the United States, Britain, and Australia. Among the spies identified were Julius Rosenberg, physicist Klaus Fuchs (who passed atomic secrets to the Soviets), British diplomat Donald Maclean, and hundreds of others.
The information was so sensitive that it could not be used directly in court—doing so would have revealed to the Soviets that their codes were compromised. This created agonizing situations where intelligence agencies knew individuals were spies but could not always act on the knowledge. Venona remained classified until 1995, and its revelations settled many Cold War espionage debates.