Two great sources of information on this topic: Eavesdropping on Hell: Historical Guide to Western Communications Intelligence and the Holocaust, 1939-1945
and
Official Secrets: What the Nazis Planned, What the British and Americans Knew
On July 18, 1941, less than a month after Nazi Germany invaded the Soviet Union, a radio operator at a British monitoring station copied down a transmission from the German Police Regiment Centre. The message, once decrypted, reported the execution of 1,153 “Jewish plunderers” near the town of Slonim in Belarus. It was not the last such message. It was not even close.
The British had been reading German Order Police hand ciphers since September 1939. The coding system was called Double Transposition, and it was, by the standards of Enigma, relatively crude. The Germans had a habit of putting the address at the start of each message and the signature at the end. Once the codebreakers at Bletchley Park figured out who was stationed where, cracking the daily key became routine, provided they had enough intercepted text to work with. Brigadier John Tiltman made the first substantial break. By the time Churchill came to office in May 1940, the German Police Section was already turning out decrypts on a regular basis.
What nobody anticipated was what those decrypts would contain.
After June 22, 1941, when Operation Barbarossa rolled east, the radio traffic from SS and police units in the occupied Soviet territories started arriving at Bletchley Park with a new kind of content. Alongside the mundane stuff (troop movements, supply requests, personnel appointments) came reports of mass shootings. Higher SS and Police Leader Erich von dem Bach-Zelewski boasted in one transmission that executions in his region had surpassed thirty thousand. The SS Cavalry Brigade reported killing 7,819 people in the Minsk area on a single day in August. Police Regiment South logged 1,255 Jews shot near Ovruch on September 12. These were not rumours from refugees or secondhand accounts smuggled out by underground couriers. These were the killers’ own bookkeeping, transmitted by radio and read in near real-time by British intelligence.
Churchill was a famously greedy consumer of Ultra intelligence. Stewart Menzies, the head of MI6, sent him daily briefings packed with highlights from the latest decrypts. Sprinkled through those briefings were references to the massacres of Jews in Russia, anti-Jewish measures across occupied Europe, transports, deportations, camp populations. On August 24, 1941, Churchill delivered a radio broadcast denouncing Nazi atrocities in the Soviet Union. He spoke of “scores of thousands” of Russian patriots being executed, of “whole districts” being exterminated. He even named the German police troops responsible, a detail so precise it could only have come from signals intelligence.
He said nothing about Jews.
The intelligence summary he received three days before the broadcast had mentioned Jewish executions but placed them in a broader context of Nazi measures to reduce the Soviet population. The German reports themselves muddied the picture on purpose. Bach-Zelewski labelled his victims “Jewish plunderers” or “Bolshevists” or “partisans,” possibly on Himmler’s instructions. This deliberate vagueness created confusion in London. Were these military reprisals, or something else entirely? Some British officials clearly preferred not to ask the question too loudly.
Victor Cavendish-Bentinck, chairman of the Joint Intelligence Committee, had access to the police and SS decodes throughout the war. He knew, by 1943, that the Nazis had a policy of eliminating Jewish women, children, and the elderly, sparing only men fit for forced labour. Simple arithmetic should have told him the death toll ran into the millions, and Polish and Jewish sources were saying exactly that. His response was to dismiss those sources as unreliable, accusing both groups of trying to “stoke us up.” After the war, Cavendish-Bentinck visited Auschwitz. When he returned to tell his Foreign Office colleagues the Nazis had murdered millions, they still didn’t believe him.
The story gets worse when you look at what happened to the decrypts themselves.
In late spring 1996, historian Richard Breitman sat in the National Archives in College Park, Maryland, working his way through the NSA Historical Cryptographic Collection. The NSA had recently declassified some 1.3 million pages of wartime material, stored across 1,440 archive boxes. In box 1386, Breitman found an extensive set of German Order Police decodes from 1941-42, with a handful from 1943. The documents were in German, or what British analysts had managed to decipher of it. They showed not only what the Nazis had reported but exactly when British codebreakers had deciphered each transmission and which offices in London received the intelligence. As original documentation of Nazi killing operations and simultaneous proof of British knowledge of those operations, the decodes were, in Breitman’s assessment, unique.
Portions of this story had been known in outline since F.H. Hinsley published his official history of British wartime intelligence in the early 1980s. But Hinsley’s treatment of the Order Police occupies three pages in an appendix. “Jews” did not appear in his index. He tallied some incidents, yes, but his count was incomplete. The full picture, which the 1996 declassification began to reveal, showed something scholars had underestimated for decades: the massive involvement of ordinary Order Police battalions in the mass shootings. These were not elite SS fanatics selected for ideological purity. Many of their commanders had joined the Nazi Party after Hitler came to power, apparently because membership was useful for career advancement. The rank and file were probably even less political. They shot people anyway.
By war’s end, Allied codebreakers had intercepted and decrypted somewhere between 700 and 900 individual messages bearing directly on the Holocaust. That sounds like a lot. It wasn’t nearly enough. The decrypts gave the British and Americans a window into the killing, but the window was small, it was dirty, and for long stretches, somebody had drawn the curtain.
Let me walk through what they actually had.
The Eastern Front Massacres
The earliest and most damning intelligence came from German Order Police radio messages, which Bletchley Park had been reading since late 1939. When Germany invaded the Soviet Union in June 1941, the Order Police adopted a new key and frequency for the Eastern Front, which doubled the codebreakers’ workload. They cracked less than half the transmissions on a given day, and during August 1941, when the Germans switched to two keys per day, the success rate dropped to about twenty-five percent.
What they did read was horrifying enough. Dozens of open reports of mass killings poured in from the Higher SS and Police Leaders. On August 7, 1941, Erich von dem Bach-Zelewski, the Higher SS and Police Leader for Army Group Center, boasted by radio that executions in his region had surpassed thirty thousand. A Bletchley Park analyst attached this comment to one of the decrypts: “The tone of this message suggests that word has gone out that a definite decrease in the total population of Russia would be welcome in high quarters and that the leaders of the three sectors stand somewhat in competition with each other as to their scores.” The War Office fired back a sharp retort that this interpretation rested on “a minimum of evidence.”
So even inside the intelligence establishment, there was a fight over what these messages actually meant.
The very first intercept mentioning a massacre was probably the July 18, 1941 message from Police Regiment Center, reporting the execution of 1,153 “Jewish plunderers” near Slonim in Belorussia. From there, the reports kept coming, with victims described variously as “Jews,” “Jewish plunderers,” “Jewish Bolshevists,” and “Russian soldiers.” The SS Cavalry Brigade alone reported 7,819 killed in the Minsk area on August 7. Police Regiment South reported 1,255 Jews shot near Ovruch on September 12. These were just the ones British codebreakers managed to read; the actual killing was orders of magnitude worse.
Churchill saw this material. On August 24, 1941, he gave a radio speech about German atrocities on the Eastern Front, drawing directly from the police decrypts. He mentioned that “whole districts are being exterminated,” but he said nothing about Jews being specific targets. Whether this was because he hadn’t yet grasped the racial dimension, or because he was being strategically vague to protect the source, remains debated.
The speech may have been a catastrophic mistake in one narrow sense. On September 12, Kurt Daluege, the commander of the Order Police, ordered all units to stop radioing reports of executions. In November 1941, the police changed their cipher system. British cryptologists believed Churchill’s broadcast triggered both changes. Ironically, the new cipher, a double Playfair system, proved easier for Bletchley Park to break than the one it replaced. Lucky, that. It could just as easily have gone the other way.
By September 11, 1941, Nigel de Grey, deputy director of GC&CS, had seen enough. He noted on a batch of decrypts: “The fact that the Police are killing all Jews that fall into their hands should now be sufficiently well appreciated. It is not, therefore, proposed to continue reporting these butcheries unless so requested.” Whether this reflected a failure of imagination or a bureaucratic desire to look away, the effect was the same. The reporting pipeline narrowed.
The Concentration and Death Camps
The SS ran its own radio network using a version of the Enigma cipher machine. GC&CS broke this system in late 1940 and gave it the cover name “Orange.” Starting in mid-1942, intercepted messages from the camps began flowing into Bletchley Park. These were mostly formatted administrative reports: daily population counts of slave labourers, broken down by nationality and ethnic group. Auschwitz, identified in radio traffic by the single letter “F,” reported totals for the beginning and end of each day across eight columns: total at start, increase, decrease, total at end, Jews, Poles, unknown, and Russians.
Here was the critical gap. These reports tallied only the registered prisoner population. They said nothing about the people who arrived on transports and were sent directly to the gas chambers. The SS kept that information off the radio. Whether this was a deliberate security measure or simply the way the bureaucratic reporting was structured, the result was that analysts at Bletchley Park, relying solely on SS and police decrypts, could not build an explicit intelligence picture of the mass extermination at Auschwitz-Birkenau.
They could, and should, have inferred it. British intelligence was also reading German railway ciphers from early in the war. A Railway Research Service within the Ministry of Economic Warfare had been analysing German rail transportation since February 1941. The railway decodes (which apparently remain classified to this day) would have shown massive shipments of Jews to Auschwitz. The camp population data showed that Jews were not arriving in numbers anywhere close to what the transports must have been carrying, and they were not departing. As Richard Breitman put it in Official Secrets: “Had Auschwitz become one of the largest cities of Europe? There was only one logical conclusion about the fate of Jews transported there.” Specific British intelligence conclusions about Auschwitz-Birkenau either have not survived or remain classified.
One recently discovered decrypt laid out in numbers that should have been impossible to misread. On January 11, 1943, the SS leader in Lublin radioed Berlin with the totals from Operation Reinhard: the number of Jews killed at Belzec, Sobibor, Treblinka, and Lublin for both the last two weeks of December 1942 and for the full year. The total for 1942 was 1,274,166 people. The British analysts who decrypted this message appear to have missed its significance entirely. The reference to “Einsatz Reinhardt” followed by a column of numbers apparently meant nothing to them, or at least nothing that prompted action.
There were other telling fragments. A June 1942 WVHA instruction informed camp commanders that reports of executions were no longer “top secret” but merely “secret,” meaning they could now be sent by radio. The implication, as Breitman noted, was that something considerably more sensitive than ordinary shootings was happening at the camps. Traces of meetings between Eichmann and extermination camp officials showed up in garbled decodes. In October 1943, British intelligence learned from a decoded SD message that seven hundred Jews had just broken out of Sobibor and that countermeasures were underway. The message even gave Sobibor’s precise location: five kilometres from the Bug River, between Cholm and Wlodawa. This kind of operational detail could have been used to plan an intervention. Nobody moved.
The Diplomatic Traffic
Beyond the police and SS decrypts, the Americans and British were reading diplomatic cables from neutral and Allied nations. Most of these were low-priority intercepts, but they occasionally carried eyewitness accounts from diplomats on the ground. The traffic picked up during the roundups and deportations in Vichy France in summer and fall 1942, and again during the destruction of Hungary’s Jews in spring and fall 1944. The Portuguese ambassador in Budapest sent cables so harrowing that even the distant professionals who processed them could not maintain their objectivity. Other intercepted messages chronicle the futile efforts of neutral diplomats trying to organise rescue operations that went nowhere.
Japanese diplomatic traffic from Berlin and Rome, read through the MAGIC program, also carried scattered information. A Japanese report from Vienna in late February 1942 mentioned that ten per cent of workers in the area were kept in concentration camps. These were fragments, breadcrumbs, not a full picture.
What They Couldn’t See
The decrypts never captured the high-level Nazi planning for the Final Solution. The Wannsee Conference of January 1942, where the bureaucratic machinery for the extermination of all European Jews was formalised, produced no radio traffic for the Allies to intercept. Orders went by courier. Plans were communicated orally. Hitler’s March 1941 directive establishing the invasion of the Soviet Union as a “war of extermination” left no trace in signals intelligence. The arrangements between the army and SS to implement that policy were made face-to-face.
The Gestapo’s most secret Enigma key, which would have contained information about the Einsatzgruppen and deportations, was never broken by Bletchley Park. Other Gestapo and SD codes were apparently broken, but those records have not been released.
So the intelligence that reached the Allies was always after the fact, incomplete, and filtered through a bureaucracy with strong institutional reasons to look the other way.
Why Nothing Was Done
The overriding reason, the one that trumped everything else, was the security of Ultra itself. If the Germans discovered that the Allies had cracked Enigma, they could have installed new systems overnight. The Allies would have been plunged back into cryptographic darkness at a moment when Ultra was shaping the outcome of the Battle of the Atlantic, the North African campaign, and eventually the invasion of Europe. No one in authority was willing to risk that. For the codebreakers, no matter what horrors the decrypts revealed, the best security policy was to say nothing and keep reading.
There was also a propaganda concern. The Germans could, and did, use Allied statements about Jewish persecution as ammunition for their own propaganda, denying the gassing of Jews and publicising stories about “Jewish control” of the Allied war effort.
The Foreign Office’s failure to use the police decodes either during the war or in postwar trials is consistent with a broader pattern. Victor Cavendish-Bentinck, who chaired the Joint Intelligence Committee, decided that Polish and Jewish sources were unreliable because they had “an interest in exaggerating.” He wrote in 1943 that the Nazis had a policy of eliminating Jewish women, children, and the elderly while sparing men capable of labour. This partial understanding should have led him to conclude that victims numbered in the millions. It didn’t lead him anywhere useful. After the war, he visited Auschwitz in person, and when he told his Foreign Office colleagues that the Nazis had killed millions, they still didn’t believe him.
Churchill’s Secret Intelligence Service stopped reporting executions of Jews to him in mid-September 1941, barely two months after the decrypts first revealed the massacres. For the next year and more, Churchill apparently received no reports about the fate of European Jews, having delegated the matter to Eden and the Foreign Office. The Foreign Office showed little inclination to believe the worst reports, let alone recommend doing anything about them.
Telford Taylor, the American intelligence officer who eventually prosecuted Nazis at Nuremberg, didn’t begin receiving copies of the police decodes until 1944. He later wrote in his memoirs that he first became aware of the Holocaust through witnesses and documents at Nuremberg in the second half of 1945. A man who had spent the war in Allied signals intelligence, and who would go on to prosecute the perpetrators, claimed he learned about the thing itself only after it was over.
The full documentary record didn’t become available to the public until the mid-1990s, when the NSA declassified 1.3 million pages of wartime material and Richard Breitman found the German Order Police decodes in box 1386 of the NSA Historical Cryptographic Collection. The British released their complete set of police decrypts to the Public Record Office in May 1997. As of Breitman’s writing, NSA still held several hundred thousand more pages of classified World War II documents, and key British files on figures like Himmler and Kaltenbrunner remained withheld.
The decrypts told the Allies enough. They told them that Jews were being shot by the thousands on the Eastern Front from the summer of 1941. They told them that the camp populations were shifting in ways that only mass killing could explain. They told them the precise locations of death camps and the numbers being processed through them. A single intercept from January 1943 told them that over 1.27 million people had been killed in the Operation Reinhard camps in a single year. The information was there. What was missing was the will to act on it, and, it must be said, any clear idea of what acting on it would have looked like in the middle of the largest war in human history.
What makes the decrypts so damning is not just what they documented, but what was done with that documentation. Nothing, mostly. The intelligence was filed. It was classified. At the war’s end, Bletchley Park commissioned an internal history, and the analyst who wrote the section on German police decodes added a note at the bottom of his appendix suggesting that “Jews” and “Bolshevists” were probably “convenient categories to account for any kind of execution.” This was written after Bergen-Belsen, after Dachau, after the Nuremberg trials had already begun. Even then, someone at Bletchley Park couldn’t quite bring himself to say what the decrypts plainly showed.
The British never used the police decodes for war crimes prosecutions. The Allied Control Council placed Order Police lieutenants and below outside the scope of mandatory denazification. Nobody at Nuremberg saw the intercepts. The Americans who took over prosecution planning didn’t receive copies of the police decodes until 1944, and even then, the chief American liaison later wrote that he first became aware of the Holocaust through witnesses and documents at Nuremberg in the second half of 1945, not through signals intelligence.
So the messages sat in classified files for half a century, gathering dust, while historians pieced together the story of the Holocaust from every other available source. The intercepts that could have confirmed the killing, identified the killers, and pinpointed the locations were locked away until the mid-1990s. The codebreakers knew. The intelligence officers knew. Churchill knew, at least in part. What they knew, and when, and what they chose to do about it (which was very little) remains one of the war’s most unsettling questions. The signals were there. They were just never acted upon.