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Who was the greatest spy of World War II? By that, I don’t mean the most colorful the most James Bondlike, the most romantic, or the most skilled trade craftsman. I mean the most effective. I mean the most affected the course of the war.

Some will say it was Richard Sorge—the motorcycle-riding, womanizing, blue-eyed German secret Communist in Japan who concealed his spying by freelancing as a newspaper and magazine correspondent. After Adolf Hitler attacked Russia, Sorge had to learn what Japan would do. Would the empire move north to strike its traditional enemy from behind while in a life-and-death struggle, or would Japan move south, toward oil and the white man’s empire in Asia? And indeed, at the end of September 1941, Sorge reported, “The Soviet Far East can be considered safe from Japanese attack.”

But his achievement was not unalloyed. Russian codebreaking had revealed that Japanese troops were not moving north. Moreover, necessity drove Josef Stalin to pull troops out of the Far East to fight the Germans approaching Moscow. Sorge’s influence is thus at best clouded, so he therefore cannot stand clearly as the greatest spy of World War II.

How about Juan Pujol, code-named “Garbo”? This thirty-two-year-old anti-Fascist Spaniard had promised the Germans that he could provide them information from England, where he would be going for business reasons. Once there, he convinced the British that he could serve them as a double agent. And indeed, under their tutelage he fed the Germans information that was accurate but insignificant, or assumed to be already known to them.

Then, when their trust had solidified, he radioed them the big lie: that the Anglo-American assault on Normandy served merely as a feint for the main invasion, which would come farther north, opposite Dover, in France’s Pas de Calais. The Germans believed him. His report sped to Berlin and back down to the Western Front—where four divisions were held in northern France to await what the Germans thought would be the real assault.

This certainly contributed to the success of the invasion, and may even be said to have driven the final nail into the deception plan. However, it was indeed only one nail in a huge operation. Valuable as Garbo was, he cannot be regarded as the greatest spy of the war. At best, he confirmed a German misconception; he did not create it.

A couple of Germans defied the Gestapo and delivered information to Allen Dulles, the Office of Strategic Services spymaster in Switzerland. Hans Bernd Gisevius, a counterintelligence agent operating under cover as a vice consul in Zurich, reported, for example, that the Germans were preparing two types of missiles—later called V-1 and V-2—that enabled the Allies to understand seemingly contradictory intelligence reports.

However, this information was provided after British aerial photographs had found the installations in Peenemünde. Fritz Kolbe, code-named “George Wood,” a staffer under a high Foreign Office official, passed photographs of diplomatic messages to Dulles. They described the V-weapons, the Reich’s transportation problems, the Volkssturm, and the so-called Alpine redoubt. His biographer has called Kolbe “the most important spy of World War II.” A more sober assessment by historian Christoph Mauch says that “When it was realized how valuable the material presented by Wood really was, a good portion of its significance had already been lost.”

On the Axis side, “Cicero” holds pride of place. He was an Albanian swindler, Elyesa Bazna, who had wormed himself into a job as butler for the British ambassador to Turkey. In the fall of 1943, he copied the diplomat’s key to his safe, took secret documents from it, photographed them, and sold the films to the Germans. Turkey, a World War I ally of Germany and an old enemy of Russia, could threaten Allied control of the Mediterranean, but Allied advances precluded any significant action based on this inside information. Cicero’s information, though interesting, did not matter.

No, I believe the greatest spy was one who worked before the war but whose information affected that conflict more than any other secret agent. His name is…but that is getting ahead of my story.

In 1973 I was at Oxford University writing my dissertation on German military intelligence in World War II. My supervisor was Hugh Trevor-Roper, whose job in British intelligence during the war had been to study that very subject.

One day in the spring, I received a note from an acquaintance, J. Rives Childs. A retired U.S. career ambassador who in World War I had broken codes for the American Expeditionary Forces, Childs had lent me his papers from that work when I was writing The Codebreakers. Later we met in New York and in Paris.

Childs was now inviting me to lunch with some English friends near Oxford. During the meal, the conversation turned to a new French book by General Gustave Bertrand, a retired French intelligence officer. He had run a spy early in the 1930s who provided information about the main German cipher machine that led to its solution. This machine, called Enigma, was put into service in the German navy in 1926 and the German army in 1928; it concealed many of the Wehrmacht’s transmitted secrets throughout World War II. Bertrand told the story of the spy in his book Enigma, ou la Plus Grande Enigme de la Guerre 1939-1945.

If this man had indeed provided the information that enabled the Allies to crack Enigma messages and so gain access to key German military information, such as U-boat orders, he would indeed be the greatest spy of World War II. But who was he?

I bought Bertrand’s book. It detailed how in October 1932 an employee of the German Defense Ministry’s Chiffrierstelle, or Cipher Center, “offered his services” to the French. It provided much color about Bertrand’s eighteen meetings with the spy in cities and resorts throughout Europe, and some details about the information he supplied, at first about Enigma, later about other German cryptosystems and the work of the Forschungsamt, Hermann Göring’s wiretapping agency, to which the spy transferred in 1934.

Even with this help, however, the French cryptanalysts could not solve the Enigma cryptograms. Bertrand related how, with his supervisors’ permission, he gave the spy’s information to other countries concerned about German revanchism: Britain, Czechoslovakia, and Poland. Alone among all these countries—and “thanks to the information” from the agent, as the head of the Polish cryptanalysts acknowledged to Bertrand—Poland reconstructed the machine in December 1932 and read messages enciphered in it.

At a meeting in Warsaw in July 1939, the Poles told the British and French how three young Polish cryptanalysts had solved Enigma. Poland gave its friends two copies each of the Enigma. Both used them after the war broke out—France less successfully than Britain. But throughout his story, Bertrand used only the spy’s cover name—the initials H.E., pronounced in French asché. He never provided the real name.

There the matter rested until the summer of 1974, when retired Royal Air Force Group Captain Frederick William Winterbotham obtained permission to disclose that the Allies had been intercepting, solving, and exploiting German Enigma-enciphered messages, thus helping them win World War II. His book The Ultra Secret exploded in headlines in the British press. The story was utterly surprising and extremely significant. No previous writings in English, French, or German had even hinted at it. Churchill had purged his memoirs of any references to intercepts and solutions. The official historians had not been told about Ultra—the interception, solution, and exploitation of coded German messages during World War II.

Winterbotham’s revelations showed how much Allied commanders knew about German plans, supplies, order of battle, and much more—and thus how the Allies had won the war more quickly and more cheaply in men and materiel than they otherwise might have done. Some journalists and historians said that the Ultra revelations would require a total rewriting of the war.

This was hyperbole. At best, knowledge of Ultra helped explain why some things happened; it did not change the events. Still, as literary critic George Steiner has said, the breaking and exploitation of the German cryptograms was Britain’s greatest achievement of the twentieth century.

I do not know why Winterbotham, who ran the system of extremely limited distribution of solved intercepts to military commanders, was allowed to reveal this tightly held secret before any of the other tens of thousands of World War II codebreakers. Many of them had been more central to the production and employment of this vital intelligence than he was, and had kept the secret for decades.

I did learn later why Her Majesty’s government had decided to release the story after three decades of silence. At the end of European hostilities in 1945, the Allies gathered up thousands of Enigma machines from the Luftwaffe, the Kriegsmarine, the SS, the land forces of the former Wehrmacht, even the railroad administration. Soon decolonization began, and the newly independent members of the British Commonwealth—India, Pakistan, Nigeria, Ghana, and the others—clamored for cryptosystems for their diplomats and their soldiers. Only one firm in the world, the Swede Boris Hagelin’s Cryptoteknik, then sold cipher machines in any quantity. Some of the new countries bought these machines. Others turned to the former mother country for help. Britain offered them the surplus Enigmas, systems more secure than Hagelin’s.

Now, officials in these countries were not so naive as to think that, if Britain was giving them cipher machines, it could not read them. But they were less concerned about Britain than about their neighbors—Pakistan was worried about India, for example. Those countries, many of them in the Third World, did not have and could not buy the cryptanalytic expertise to break Enigma. Many countries purchased the code machines.

However, the Enigma was electromechanical. It had rotors and pawls and contacts. By the early 1970s, the machines had worn out. Countries replaced them with more advanced cryptosystems, often electronic. At this point, Britain no longer had to keep secret the fact that it could read Enigma-encrypted messages.

Sir Joe Hooper, a former head of Britain’s codebreaking agency, explained all this to me. We were standing in an ornate hall in Britain’s Foreign and Commonwealth Office, the same building in which Foreign Secretary Sir Edward Grey, watching the lamplighters in Whitehall on the eve of World War I, said: “The lamps are going out all over Europe. We shall not see them lit again in our lifetime.” For me, as a historian of cryptology, the moment was almost as historic.

Winterbotham’s book amazed, fascinated— and chagrined—me perhaps more than anybody else. My book The Codebreakers, published in 1967, had nothing about the biggest cryptologic story of all time. I knew that ten thousand people worked at Bletchley Park, the British codebreaking center, but I could not find out what they did, or what effect they had had. I knew no one who might talk, and although while researching that book I had written Winston Churchill and Dwight Eisenhower, asking what effect codebreaking had had on the war, neither had responded.

Later in the summer of 1974, my wife and son were vacationing in Jesolo, near Venice, and I was to join them. I wanted to learn more about the Enigma solution. France was on the way to Italy, and General Bertrand agreed to let me interview him. I met him on July 12 in the Hôtel la Tour de l’Esquillon, atop a cliff on the edge of the Mediterranean in Théoule-sur-Mer, near Nice. Among other details, such as what French cryptanalysis had achieved in the first months of the war, I wanted to learn the identity of the spy.

But although Bertrand was helpful on other matters, saying that the man had been caught and shot shortly before the war and that his brother was a famous panzer general, he declined to name him. I excused myself and placed a telephone call to Walther Seifert in Osnabrück, West Germany. I had interviewed him twice for my book Hitler’s Spies. Seifert, short, pug-nosed, cigar-smoking, had been chief of evaluation of the Forschungsamt, the agency in which Bertrand’s spy had later worked. The Forschungsamt had left few documentary traces in Nazi Germany, and I needed to learn about it. Seifert had been extremely interested in his work and good at it. He and I got along very well; they were among my most productive interviews.

When he answered my telephone call from Théoule, I asked him whether any employees of the Defense Ministry’s Chiffrierstelle and then the Forschungsamt had ever been arrested for betraying secret information. He first gave the name of one Plaas, shot in 1944, but then reconsidered and observed that Plaas had never served in the Chiffrierstelle.

Then he said that Hans-Thilo Schmidt, a Forschungsamt employee under him, had worked in the Chiffrierstelle. He said that around 1942 he was caught and shot for giving information to the enemy. He had done it for money and women. Seifert characterized him as a Waschlappen—a dishrag, a weakling. And he said that his brother was a panzer general! (This was Colonel-General Rudolf Schmidt, who commanded the XXXIX Panzer Corps in Russia and then replaced General Heinz Guderian, Germany’s famed apostle of tank warfare, as commander of the Second Panzer Army. He was a favorite of Hitler’s.)

I thanked Seifert, hung up, returned to Bertrand, and presented this information to him. I hoped that it would shock him into a confirmation. It didn’t. He declined to confirm or deny the identification, saying that the man’s wife and children were still alive and should not be exposed to this further mortification.

A few weeks later, on August 7, Seifert typed out Schmidt’s name in a letter and added that he was a member of the Nazi Party. I checked the party records in the Berlin Document Center, an archive under American control, and found Hans-Thilo Schmidt’s membership card. Born May 13, 1888, he had joined the party on December 1, 1931, as member number 738,736.

Then, obtaining the service record of his brother, I learned the names of his parents—his father was a “Professor Doktor,” a prestigious title in Germany, and his mother was a baroness. That record also showed that Rudolf, a signals officer in World War I, had as a captain served from 1926 to 1928 as head of the Chiffrierstelle, the unit that both broke foreign codes and devised and authorized cryptosystems for the German army. In depression- and inflation-ridden Germany, he had given his younger brother a job in that unit.

I now knew the identity of the greatest spy of World War II. But I had no occasion to use it. Then, when Winterbotham’s book was published in America later in 1974, I reviewed it for The New York Times. I praised it, saying that the British—and later American—codebreaking was “the greatest secret of World War II after the atom bomb.” I told the story of the spy and gave his name. It was the first time he had been publicly identified, and I believe all subsequent references may be traced to this statement.

I sent a copy of the review to Bertrand, thinking it would interest him. Back came a furious note saying, “I will not hide from you that I am very angry at the indiscretion that you have committed in unveiling the name of the employee of the Cipher Center, which I had always carefully concealed, since his brother and his wife (as well as his children) are still living.” Bertrand thus inadvertently confirmed that the identification was correct.

Actually, the brother had died in 1957; I don’t know about the widow, but a daughter still lived. Further confirmation came, I discovered later, from two German sources. A 1967 article by the German intelligence historian Gert Buchheit stated that “Thilo Schmidt” had delivered some details about Enigma to the French; he expanded this in his 1975 book Spionage in Zwei Weltkriegen. And the diaries of Nazi propaganda minister Josef Goebbels, published in partial English translation in 1948, mention in the entry for May 10, 1943, the arrest for treason of the brother of Colonel-General Schmidt. (Bertrand had been wrong when he said that Schmidt had been caught before the war.) The traitor was not named, and no one seems to have paid attention to the news.

Finally, for his book on Allied codebreaking, Enigma (2001), British author Hugh Sebag-Montefiore amazingly tracked down the daughter of a man whose family name was Schmidt. She told of a father who seduced his housemaids, somewhat substantiating the statement that he had betrayed his country for money and women. He thus filled out the human picture of the man who is, in my view, not only the greatest spy of World War II but also possibly the most important spy of all time.

Schmidt’s contribution, however, tells only half the story of the Enigma solution. How did the Poles achieve it when the British and French, who also had that information, had failed? The story again revolves around Oxford.

Around the turn of 1973-1974, the head of Oxford’s faculty of modern history asked me to help another doctoral student with his dissertation on intelligence during the Napoleonic wars. The student, Alfred Piechowiak, met me in February at my house. I offered what little help I could, and in conversation then or later he said that his father, a retired lieutenant colonel in the Polish army, had been in intelligence in World War II and had some papers dealing with codebreaking. Naturally, I asked if I could see them. He readily assented, and on July 26 we went to their house in South Hinksey, a suburb of Oxford.

They gave me a thirty-two-page purple carbon copy of an anonymous typescript titled “Enigma, 1930–1940.” It was in Polish and heavily mathematical. I could understand none of it, but Piechowiak orally translated part of it for me. With his permission, I photocopied it.

By then Tadeusz Lisicki, a retired colonel living in England, had contacted me, probably because the Enigma story was breaking and he knew I had written about cryptology. He told me that Henryk Zygalski, one of the three original solvers of Enigma, was living in England. On July 29, Lisicki and I interviewed Zygalski at his home in Liss, some 30 miles southwest of London. I was thrilled to meet a man who had contributed to a cryptanalysis that had had such far-reaching effects. He provided me not with technical details, which in any event would be foggy sixty years after the event, but with color: where the cryptanalysts worked, in particular.

He also wrote out for me the name and address in Warsaw of his former colleague, Marian Rejewski, who turned out to be the chief solver of the Enigma and the author of the anonymous typescript. Besides Zygalski, Rejewski was the only survivor of the three original cryptanalysts.

Lisicki identified the corrections on the typescript as in Rejewski’s handwriting. Eventually, I believe through Lisicki, it reached a wider audience. This contributed to explaining why Poland had been the only nation to solve the Enigma though two other countries had the information from Schmidt.

One reason was that Poland had greater need: Germany thundered out threats against it more than against the others, hating that much of Poland’s land had once been held by Germany, furious that the Polish corridor divided East from West Prussia, resenting the independence of what had once been the German port of Danzig, demanding the “rectification” of its borders.

The other reason was that Poland had greater cryptanalytic ability. With more foresight than the other countries, it was the only one of the three to employ mathematicians as cryptanalysts—and only mathematics could make it possible to reconstruct the Enigma rotor’s internal wirings. Subsequently, I corresponded in German—our best mutual language—with Rejewski, filling in some details of the work.

His memorandum was later translated and published. It described how the cryptanalysis had advanced pretty well—but then had stalled. Then Rejewski received Schmidt’s information. It converted some of the unknown terms in the cryptanalytic equations to known terms, enabling Rejewski to resolve them. In a commentary, he generously acknowledged, “the intelligence material furnished to us should be regarded as having been decisive to the solution of the machine.” Thus he independently implied what I had already concluded: that Hans-Thilo Schmidt was the most important spy of World War II.

This story cannot be completed without mentioning two remarkable coincidences. One is that author Sebag-Montefiore’s great-great-grandfather, Sir Herbert Leon, had once owned Bletchley Park, the home of British World War II codebreaking.

The other is the most exquisite irony in intelligence history. As head of the Chiffrierstelle, Rudolf Schmidt approved for use the Enigma cipher machine that his brother later betrayed!

Originally published in the Autumn 2007 issue of Military History Quarterly. To subscribe, click here