What did more damage—Philby’s treachery or the subsequent obsession among spy officials with preventing future Philbys? Illustration by Rory Kurtz; courtesy of Bettmann / Corbis

When Kim Philby decided that he wanted to join the British Secret Intelligence Service, he “dropped a few hints here and there,” as he later recalled, and waited patiently. Philby had attended Trinity College, Cambridge, and his father had been in the Foreign Service. He had the right accent. It was the late nineteen-thirties, when the British class system was still firmly in place, and a formal application wasn’t necessary. On a train to London, Philby found himself in the first-class compartment with a journalist named Hester Harriet Marsden-Smedley, who was of that same small world. She looked him over and said that she would make a few inquiries on his behalf. Then he got a call from someone at the War Department, and was invited to tea at St. Ermin’s Hotel, off St. James’s, with an imperious Tory doyenne named Sarah Algeria Marjorie Maxse. They chatted. Philby was famously charming. He had impeccable manners, a disarming stammer, and an epic capacity for alcohol. His name was passed up the line to M.I.5—the British F.B.I.—which came back with the laconic verdict “nothing recorded against.” The deputy head of the British spy service, M.I.6, had served with Philby’s father in India. “I was asked about him,” the official explained later, “and I said I knew his people.”

Once Philby joined M.I.6, he roamed its halls, gossiping and making friends. The man who controlled the “source books”—the inventory of British intelligence assets—was a red-faced ex-policeman with a crippling drinking habit. Philby would go out and get him drunk, and soon had the run of the files. He became fast friends with James Angleton, who later rose to the head of counterintelligence at the C.I.A. The two of them served together in Washington, and had long boozy lunches, at which they traded the most intimate secrets. Philby was promoted to head the anti-Soviet section of M.I.6, and then became the principal liaison between the British and the U.S. intelligence agencies. “I looked around at the part-time stockbrokers and retired Indian policemen, the agreeable epicureans from the bars of White’s and Boodle’s, the jolly, conventional ex-Navy officers and the robust adventurers from the bucket shop; and then I looked at Philby,” the historian Hugh Trevor-Roper later wrote. “He alone was real. I was convinced that he was destined to head the service.”

He came close. In 1951, two of his good friends—Guy Burgess and Donald Maclean—fled to Moscow, revealing themselves to be Soviet spies. Philby’s colleagues stood by him, but he was forced to resign. He moved to Beirut to work as a correspondent for the Observer and The Economist, only to have M.I.5 launch a second investigation, in the early nineteen-sixties. Before it could be completed, Philby slipped away. In January of 1963, a car with diplomatic plates picked him up from a bar in downtown Beirut and took him to a Soviet freighter bound for Odessa. He had been, it turned out, a Soviet spy since soon after leaving Cambridge, in the mid-nineteen-thirties, dutifully feeding his K.G.B. handlers every morsel of information gleaned from his many friendships. In Moscow, he was welcomed by a congratulatory headline in an official Soviet newspaper: “HELLO MR. PHILBY.” “What it comes to is that when you look at the whole period from 1944 to 1951, the entire Western intelligence effort, which was pretty big, was what you might call minus advantage,” the C.I.A. officer Miles Copeland, Jr.—himself a close friend of Philby’s—said. “We’d have been better off doing nothing.”

“A Spy Among Friends: Kim Philby and the Great Betrayal” (Crown) is the latest in Ben Macintyre’s series on twentieth-century espionage (including the best-selling “Operation Mincemeat”). All are superb, and “A Spy Among Friends” is no exception. Macintyre gives the familiar story of Philby new life, putting the case in its full social context.

Philby’s boss was Sir Stewart Menzies, who, we are told, “rode to hounds, mixed with royalty, never missed a day at Ascot, drank a great deal, and kept his secrets buttoned up behind a small, fierce mustache. He preferred women to men and horses to both.” Menzies was an amateur at a time when his adversaries were professionals. Philby’s fellow Soviet spy Donald Maclean was a mess. But since he was a mess with the right accent and background he easily found a home in the British spy service. At one point, Macintyre says, Maclean “got drunk, smashed up the Cairo flat of two secretaries at the U.S. embassy, ripped up their underwear, and hurled a large mirror off the wall, breaking a large bath in two. He was sent home, placed under the care of a Harley Street psychiatrist, and then, amazingly, after a short period of treatment, promoted to head the American desk at the Foreign Office.”

When suspicion finally fell on Burgess, he was placed under surveillance. But this was the kind of surveillance intended for people for whom surveillance was not actually thought necessary. The “watchers” did not work weekends or evenings. They rarely left London. “Most were former police officers selected for their sharp eyesight, good hearing, and average height,” Macintyre writes. “They were expected to dress in trilby hats and raincoats and communicated with one another by hand signals. . . . They looked, in short, exactly like surveillance agents.” Philby, Macintyre concludes, “existed within the inner circle of Britain’s ruling class, where mutual trust was so absolute and so unquestioned that there was no need for elaborate security precautions. They were all part of the same family.”

Within a few years, that complacency was shattered. In 1961, M.I.6 learned that one of its operatives, George Blake, was a Soviet spy who had exposed scores of British and American agents in Eastern Europe. In 1962, a naval attaché by the name of John Vassal was found to have given away a treasure trove of British military secrets to the K.G.B. In 1963, the Profumo scandal raised the possibility that naval secrets had been passed to the Soviets by a prominent politician. In 1964, the Queen’s art adviser, Sir Anthony Blunt, confessed to having been a Soviet spy since his twenties. “A Spy Among Friends” is the portrait of an England suddenly vulnerable to its enemies. It makes for riveting reading, except that it leaves out a crucial part of the story, which is what happened next.

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?