While driving out of the American embassy in Moscow one evening in 1978, the chief of the CIA’s Moscow station heard a knock on his car window. A man on the curb handed him an envelope whose contents stunned U.S. intelligence: details of top-secret Soviet research and developments in military technology that were totally unknown to the United States. In the years that followed, the man, Adolf Tolkachev, an engineer in a Soviet military design bureau, used his high-level access to hand over tens of thousands of pages of technical secrets.