On April 30, 1943, a fisherman came across a badly decomposed corpse floating in the water off the coast of Huelva, in southwestern Spain. The body was of an adult male dressed in a trenchcoat, a uniform, and boots, with a black attaché case chained to his waist. His wallet identified him as Major William Martin, of the Royal Marines. The Spanish authorities called in the local British vice-consul, Francis Haselden, and in his presence opened the attaché case, revealing an official-looking military envelope. The Spaniards offered the case and its contents to Haselden. But Haselden declined, requesting that the handover go through formal channels—an odd decision, in retrospect, since, in the days that followed, British authorities in London sent a series of increasingly frantic messages to Spain asking the whereabouts of Major Martin’s briefcase.

In the months before the invasion of Sicily, British spies fooled German spies with a caper inspired by a detective novel. Illustration by John Ritter

It did not take long for word of the downed officer to make its way to German intelligence agents in the region. Spain was a neutral country, but much of its military was pro-German, and the Nazis found an officer in the Spanish general staff who was willing to help. A thin metal rod was inserted into the envelope; the documents were then wound around it and slid out through a gap, without disturbing the envelope’s seals. What the officer discovered was astounding. Major Martin was a courier, carrying a personal letter from Lieutenant General Archibald Nye, the vice-chief of the Imperial General Staff, in London, to General Harold Alexander, the senior British officer under Eisenhower in Tunisia. Nye’s letter spelled out what Allied intentions were in southern Europe. American and British forces planned to cross the Mediterranean from their positions in North Africa, and launch an attack on German-held Greece and Sardinia. Hitler transferred a Panzer division from France to the Peloponnese, in Greece, and the German military command sent an urgent message to the head of its forces in the region: “The measures to be taken in Sardinia and the Peloponnese have priority over any others.”

The Germans did not realize—until it was too late—that “William Martin” was a fiction. The man they took to be a high-level courier was a mentally ill vagrant who had eaten rat poison; his body had been liberated from a London morgue and dressed up in officer’s clothing. The letter was a fake, and the frantic messages between London and Madrid a carefully choreographed act. When a hundred and sixty thousand Allied troops invaded Sicily on July 10, 1943, it became clear that the Germans had fallen victim to one of the most remarkable deceptions in modern military history.

The story of Major William Martin is the subject of the British journalist Ben Macintyre’s brilliant and almost absurdly entertaining “Operation Mincemeat” (Harmony; $25.99). The cast of characters involved in Mincemeat, as the caper was called, was extraordinary, and Macintyre tells their stories with gusto. The ringleader was Ewen Montagu, the son of a wealthy Jewish banker and the brother of Ivor Montagu, a pioneer of table tennis and also, in one of the many strange footnotes to the Mincemeat case, a Soviet spy. Ewen Montagu served on the so-called Twenty Committee of the British intelligence services, and carried a briefcase full of classified documents on his bicycle as he rode to work each morning.

His partner in the endeavor was a gawky giant named Charles Cholmondeley, who lifted the toes of his size-12 feet when he walked, and, Macintyre writes, “gazed at the world through thick round spectacles, from behind a remarkable moustache fully six inches long and waxed into magnificent points.” The two men coördinated with Dudley Clarke, the head of deception for all the Mediterranean, whom Macintyre describes as “unmarried, nocturnal and allergic to children.” In 1925, Clarke organized a pageant “depicting imperial artillery down the ages, which involved two elephants, thirty-seven guns and ‘fourteen of the biggest Nigerians he could find.’ He loved uniforms, disguises and dressing up.” In 1941, British authorities had to bail him out of a Spanish jail, dressed in “high heels, lipstick, pearls, and a chic cloche hat, his hands, in long opera gloves, demurely folded in his lap. He was not supposed to even be in Spain, but in Egypt.” Macintyre, who has perfect pitch when it comes to matters of British eccentricity, reassures us, “It did his career no long-term damage.”

To fashion the container that would keep the corpse “fresh,” before it was dumped off the coast of Spain, Mincemeat’s planners turned to Charles Fraser-Smith, whom Ian Fleming is thought to have used as the model for Q in the James Bond novels. Fraser-Smith was the inventor of, among other things, garlic-flavored chocolate intended to render authentic the breath of agents dropping into France and “a compass hidden in a button that unscrewed clockwise, based on the impeccable theory that the ‘unswerving logic of the German mind’ would never guess that something might unscrew the wrong way.” The job of transporting the container to the submarine that would take it to Spain was entrusted to one of England’s leading race-car drivers, St. John (Jock) Horsfall, who, Macintyre notes, “was short-sighted and astigmatic but declined to wear spectacles.” At one point during the journey, Horsfall nearly drove into a tram stop, and then “failed to see a roundabout until too late and shot over the grass circle in the middle.”

Each stage of the deception had to be worked out in advance. Martin’s personal effects needed to be detailed enough to suggest that he was a real person, but not so detailed as to suggest that someone was trying to make him look like a real person. Cholmondeley and Montagu filled Martin’s pockets with odds and ends, including angry letters from creditors and a bill from his tailor. “Hour after hour, in the Admiralty basement, they discussed and refined this imaginary person, his likes and dislikes, his habits and hobbies, his talents and weaknesses,” Macintyre writes. “In the evening, they repaired to the Gargoyle Club, a glamorous Soho dive of which Montagu was a member, to continue the odd process of creating a man from scratch.” Francis Haselden, for his part, had to look as if he desperately wanted the briefcase back. But he couldn’t be too diligent, because he had to make sure that the Germans had a look at it first. “Here lay an additional, but crucial, consideration,” Macintyre goes on. “The Germans must be made to believe that they had gained access to the documents undetected; they should be made to assume that the British believed the Spaniards had returned the documents unopened and unread. Operation Mincemeat would only work if the Germans could be fooled into believing that the British had been fooled.” It was an impossibly complex scheme, dependent on all manner of unknowns and contingencies. What if whoever found the body didn’t notify the authorities? What if the authorities disposed of the matter so efficiently that the Germans never caught wind of it? What if the Germans saw through the ruse?

In mid-May of 1943, when Winston Churchill was in Washington, D.C., for the Trident conference, he received a telegram from the code breakers back home, who had been monitoring German military transmissions: “MINCEMEAT SWALLOWED ROD, LINE AND SINKER.” Macintyre’s “Operation Mincemeat” is part of a long line of books celebrating the cleverness of Britain’s spies during the Second World War. It is equally instructive, though, to think about Mincemeat from the perspective of the spies who found the documents and forwarded them to their superiors. The things that spies do can help win battles that might otherwise have been lost. But they can also help lose battles that might otherwise have been won.

In early 1943, long before Major Martin’s body washed up onshore, the German military had begun to think hard about Allied intentions in southern Europe. The Allies had won control of North Africa from the Germans, and were clearly intending to cross the Mediterranean. But where would they attack? One school of thought said Sardinia. It was lightly defended and difficult to reinforce. The Allies could mount an invasion of the island relatively quickly. It would be ideal for bombing operations against southern Germany, and Italy’s industrial hub in the Po Valley, but it didn’t have sufficient harbors or beaches to allow for a large number of ground troops to land. Sicily did. It was also close enough to North Africa to be within striking distance of Allied short-range fighter planes, and a successful invasion of Sicily had the potential to knock the Italians out of the war.

Mussolini was in the Sicily camp, as was Field Marshal Kesselring, who headed up all German forces in the Mediterranean. In the Italian Commando Supremo, most people picked Sardinia, however, as did a number of senior officers in the German Navy and Air Force. Meanwhile, Hitler and the Oberkommando der Wehrmacht—the German armed-forces High Command—had a third candidate. They thought that the Allies were most likely to strike at Greece and the Balkans, given the Balkans’ crucial role in supplying the German war effort with raw materials such as oil, bauxite, and copper. And Greece was far more vulnerable to attack than Italy. As the historians Samuel Mitcham and Friedrich von Stauffenberg have pointed out, “in Greece all Axis reinforcements and supplies would have to be shipped over a single rail line of limited capacity, running for 1,300 kilometers (more than 800 miles) through an area vulnerable to air and partisan attack.”

All these assessments were strategic inferences from an analysis of known facts. But this kind of analysis couldn’t point to a specific target. It could only provide a range of probabilities. The intelligence provided by Major Martin’s documents was in a different category. It was marvellously specific. It said: Greece and Sardinia. But because that information washed up onshore, as opposed to being derived from the rational analysis of known facts, it was difficult to know whether it was true. As the political scientist Richard Betts has argued, in intelligence analysis there tends to be an inverse relationship between accuracy and significance, and this is the dilemma posed by the Mincemeat case.

As Macintyre observes, the informational supply chain that carried the Mincemeat documents from Huelva to Berlin was heavily corrupted. The first great enthusiast for the Mincemeat find was the head of German intelligence in Madrid, Major Karl-Erich Kühlenthal. He personally flew the documents to Berlin, along with a report testifying to their significance. But, as Macintyre writes, Kühlenthal was “a one-man espionage disaster area.” One of his prized assets was a Spaniard named Juan Pujol García, who was actually a double agent. When British code breakers looked at Kühlenthal’s messages to Berlin, they found that he routinely embellished and fictionalized his reports. According to Macintyre, Kühlenthal was “frantically eager to please, ready to pass on anything that might consolidate his reputation,” in part because he had some Jewish ancestry and was desperate not to be posted back to Germany.

When the documents arrived in Berlin, they were handed over to one of Hitler’s top intelligence analysts, a man named Alexis Baron von Roenne. Von Roenne vouched for their veracity as well. But in some respects von Roenne was even less reliable than Kühlenthal. He hated Hitler and seemed to have done everything in his power to sabotage the Nazi war effort. Before D Day, Macintyre writes, “he faithfully passed on every deception ruse fed to him, accepted the existence of every bogus unit regardless of evidence, and inflated forty-four divisions in Britain to an astonishing eighty-nine.” It is entirely possible, Macintyre suggests, that von Roenne “did not believe the Mincemeat deception for an instant.”

These are two fine examples of why the proprietary kind of information that spies purvey is so much riskier than the products of rational analysis. Rational inferences can be debated openly and widely. Secrets belong to a small assortment of individuals, and inevitably become hostage to private agendas. Kühlenthal was an advocate of the documents because he needed them to be true; von Roenne was an advocate of the documents because he suspected them to be false. In neither case did the audiences for their assessments have an inkling about their private motivations. As Harold Wilensky wrote in his classic work “Organizational Intelligence” (1967), “The more secrecy, the smaller the intelligent audience, the less systematic the distribution and indexing of research, the greater the anonymity of authorship, and the more intolerant the attitude toward deviant views.” Wilensky had the Bay of Pigs debacle in mind when he wrote that. But it could just as easily have applied to any number of instances since, including the private channels of “intelligence” used by members of the Bush Administration to convince themselves that Saddam Hussein had weapons of mass destruction.