In August 1948, a senior editor at Time magazine named Whittaker Chambers sat before the House Un-American Activities Committee and made a claim that would consume American politics for the next two years: that a respected State Department official named Alger Hiss had, a decade earlier, been a fellow member of an underground Communist cell in Washington.

In October 1986, President Ronald Reagan and Soviet General Secretary Mikhail Gorbachev met in Reykjavik, Iceland, for what both sides initially described as a preparatory meeting ahead of a larger summit. Behind the scenes, both leaders arrived carrying intelligence assessments that shaped, and ultimately nearly derailed, the encounter.

In 1952, two brothers, Morris and Jack Childs, both longtime senior officials within the Communist Party USA, approached the FBI and quietly agreed to become informants. What followed was one of the longest-running and most successful counterintelligence operations in FBI history, code-named Operation SOLO, which continued for more than a quarter century without ever being discovered by either the party or Soviet intelligence.

In August 1985, a senior KGB officer named Vitaly Yurchenko, then serving in Soviet security assignments abroad, approached American officials in Rome and asked for asylum. He was, at the time, one of the highest-ranking KGB officers ever to defect to the United States, and CIA officers who debriefed him afterward considered his intelligence genuinely valuable.

Within the KGB's foreign intelligence directorate, a division known internally as Line X operated for decades with a specific and almost bureaucratic mission: acquiring Western scientific and technological secrets that the Soviet Union's own research institutions were struggling to develop independently. Unlike the popular image of Cold War espionage centered on political or military secrets, Line X dealt in something more mundane and, in its way, more systemic: semiconductors, machine tooling, aerospace engineering, computing, and industrial manufacturing processes.

In the early 1930s, a Austrian-born intellectual named Arnold Deutsch arrived in London, ostensibly to pursue academic work, holding a doctorate in chemistry from the University of Vienna. In reality, Deutsch was an experienced Soviet intelligence officer for the NKVD, operating under the cover name Otto, and his assignment was one of the most consequential in the history of Soviet espionage: to identify and recruit talented young people at Britain's elite universities, long before they held any position worth spying for, and cultivate them over years into ideologically committed agents.

At the end of the Second World War, Vienna, like Berlin, was divided into occupation zones controlled by the four Allied powers: the United States, Britain, France, and the Soviet Union. Unlike Berlin, Vienna's very center, the historic inner city, was placed under joint four-power control, meaning representatives of all four militaries patrolled the same streets together, in the same jeeps, for a full decade, from 1945 until the Austrian State Treaty restored full sovereignty in 1955.

In 1960, a colonel in Soviet military intelligence, the GRU, made contact with Western officials, first approaching American students in Moscow, then British businessman Greville Wynne, offering to spy for the West. His name was Oleg Penkovsky, and his motives, as he later explained to his handlers, combined disillusionment with the Soviet system and a genuine fear that its leadership was dangerously reckless with nuclear weapons.