Former FBI agent Robert Hanssen spent over 20 years spying for the Russians
During the Cold War, the United States and the Soviet Union put plenty of resources into espionage. Both sides had several moles that gathered information about the inner workings of their respective intelligence agencies. The worst security breach in American history came from the least expected source.
Robert Hanssen first began working for the FBI in 1976. In 1979 he approached the Soviets with an offer to sell secret information. He disclosed several of the U.S.’s moles within the KGB as well as top-secret military and counterintelligence strategies. He escaped notice until 2001 when he was arrested and tried.