Just before Christmas in 1961, a KGB major named Anatoliy Golitsyn defected to the West after turning up unannounced at the U.S. Embassy in Helsinki. He came bearing secrets, revealing to his slightly bewildered hosts that Western intelligence agencies had been penetrated for decades by rather excellent Soviet spies. At the center of this effort was a network of British communist sympathizers who’d attended Cambridge University during the 1930s and had subsequently attained prominence within the organs of British statecraft. All these years later, it is still hard to know the full extent of their treachery. What is certain is that throughout the corridors of power in London and Washington, the eyes and ears of comrade Stalin were everywhere. So total was his access to information that, in the years following World War II, when the former allies were sliding toward confrontation, the Soviet leader was reading the correspondence between the British prime minister and the U.S. president in real time.