Malcolm Gladwell at The New Yorker:
In December of 1961, a high-ranking K.G.B. agent knocked on the door of the U.S. Embassy in Helsinki, asking for asylum. His name was Antoliy Golitsyn, and he had a remarkable secret to share. There had existed within the British intelligence service, he said, a “ring of five”—all of whom knew one another and all of whom had been recruited by the Soviets in the nineteen-thirties. Burgess and Maclean, who had decamped to Moscow a decade earlier, were No. 1 and No. 2. The art historian Anthony Blunt had been under suspicion by M.I.5 for some time. He was No. 3. No. 4 sounded a lot like Philby: that was why M.I.5 rekindled its investigation of him shortly thereafter. But who was the fifth? When Philby managed to escape to Moscow, concern grew. Had the mysterious fifth man tipped him off?
Within the espionage world, Golitsyn was a deeply divisive figure. Some suspected that he was a fabulist, who embroidered his accounts of K.G.B. secrets in order to extend his usefulness to Western intelligence. Two people remained firmly convinced of Golitsyn’s bona fides, however. The first was Philby’s lunchmate at the C.I.A., James Angleton. The news about Philby convinced Angleton that the C.I.A. must be riven with moles as well, and he set off on a frenzied search for traitors which consumed the American intelligence community for the next decade.