9. Robert Hanssen
Robert Hanssen, a senior FBI agent, secretly worked as a double agent for Russia for over twenty years. Driven by a mix of ideology and personal grievances, he provided the Soviets with highly classified information, exposing operations and agents and causing enormous damage to U.S. national security. His betrayal is considered one of the worst intelligence disasters in American history, leading to his arrest in 2001 and a life sentence in prison.
