Seventy Years Ago This Month at Bletchley Park          

December 1941

The Whole War Changes.  Two great surprises were sprung in December 1941 that were to change the whole face of the war.  The world remembers the Japanese attack on Pearl Harbour on 7th December that initiated the war in the Pacific, but has largely forgotten the surprise of the remarkable Russian winter offensive that started the day before and which threw the Germans away from Moscow.  When the Japanese planned their attacks in the Pacific they sought victory in a short war, for once the USA had had time to bring her industrial might to bear they knew they would inevitably be ground down.  They were relying on the imminent fall of Moscow to be closely followed by the collapse of Russia, which would tie the USA as well as the UK into an immediate major war effort in Europe to resist the all-conquering Hitler.  The Japanese wanted a short war to gain them oil and mineral supplies and then intended to offer the USA a peace in the Pacific, which they believed the USA would be happy to accept.  Like Germany and Britain, they believed that Russia was on the point of collapse, but the Russian success in producing fresh and powerful armies that overwhelmed the exhausted Germans before Moscow changed the whole basis on which Japan had gambled with the launch of the Pearl Harbour attack

There had been plenty of piece-meal counter-attacks by the Russians to try to halt the German offensives, but these were easily thwarted by the German superiority in machines and fighting ability. Now the Russians mount a totally unexpected well planned and co-ordinated attack on a wide front in weather that the Germans did not believe anyone could fight in and with fresh armies the Germans did not even know existed.  For Churchill immediately, and for the British as a whole gradually over time, the horror of Pearl Harbour and the series of unmitigated disasters that followed over the next six months were tempered by the realisation that, in Russia, we had an ally who had the tenacity and will to out-fight the Germans in the field, and, in the USA, another ally with the industrial capacity to out perform all our enemies. 

The Russian Winter Offensive.  Hitler had known for some time that his final offensive to capture Moscow had “started one month too late”, and was acutely anxious about the Russian winter conditions. But he was totally unaware that the Russians, under General Zhukov, were planning a great counter-offensive. As December begins the German offensive is petering out as Hitler’s exhausted troops grind to a halt just a few miles away from the city, unable to fight in the terrible weather conditions. Russian attacks in the South have caused Hitler to move some of his reserves from the Moscow front, but the Germans believe the Russians are as exhausted as they are. On 5th December along the flanks of the German salient aimed at Moscow three Russian reserve armies, fresh from the East and totally undetected by the Germans, launch an assault at three o’clock in the morning in a ferocious blizzard with temperatures falling to -35° C.  Some 85 Russian divisions attack along a 500-mile front, with 1,700 tanks that can cope with the snow better than the German panzers because of their wide tracks. The Russians have air superiority because they are operating from airfields with heated hangars, and most of their troops are well equipped for winter warfare.  The Germans are hurled back from Moscow over 100 miles in some places, in some confusion and the immediate threat to the city is removed.  Churchill welcomes the great news “All the anti-Nazi nations, great and small, rejoiced to see the first failure of the German Blitzkrieg”.
 Unwisely Stalin then orders a general offensive all along the line. Hitler sacks his retreating generals and takes over command of the army.  Gradually his demands for no retreat have their effect and by the spring the Russians are exhausted.

Sources of Information on the Russian Front. The British are learning about the success of the Russian counter-offensive largely from Enigma decrypts, for the Russians prove very reluctant to provide information directly.  By the end of December there is ample evidence from Enigma decrypts that the Russian action in saving Moscow is not just a local defensive operation but a major offensive.  On 23rd December, no doubt in a Christmas mood, Churchill tells his war-Cabinet “it is almost a miracle that the Russian counter-attack in the centre shows no sign of slackening and that she is extending her attacks to the northern and southern fronts”. British Intelligence now begins to accept that the Russians are not going to collapse, and will keep the Germans tied up fighting on the Eastern front through at least the summer of 1942.  However, from the end of December the main source of intelligence, the Vulture Enigma key of the German Army Group Centre was to become spasmodic and virtually peter out as the German armies came to rely on newly laid land-lines.  So the main source of information has become the Luftwaffe Enigma keys, the main Red key and sometimes Mustard, the German Luftwaffe Y service key, and Kestrel, the key used in Russia for ground-air co-operation broadcast. BP is also obtaining information from the Enigma keys used by the Abwehr and SS in Russia. Soon BP will start to provide further sources of information by breaking new keys for the Luftwaffe in Russia.

Pearl Harbour and the Japanese Attacks.  That the Japanese were about to launch an attack in South East Asia was not in doubt to the USA or the UK, and a general warning had been issued some days before. The Congressional Enquiry of 1946 concluded: “The Pearl Harbour air attack was the only part of the Japanese war plan that took Washington unawares”. But, due to a chapter of errors and omissions in Washington, the specific warning to Hawaii arrives too late. The Japanese attack in two waves, with 354 aircraft from 6 carriers. All eight US battleships are hit, five being sunk together with three cruisers and three destroyers, the torpedo bombers proving very effective.  But the three US carriers based there escape, as they are at sea. And Admiral Nagumo cancels the third wave planned to attack the oil storage tanks.  The Japanese had also attacked from the sea using small submarines, but this operation is a total failure.  Meanwhile the Japanese had landed on the coast of Thailand in fact half an hour by the clock before the attack on Pearl Harbour, but a day later because of the International Date Line.  After the attack on Pearl Harbour the Allies are left in the Pacific with the three US carriers and the two British capital ships that have just reached Singapore.  Three days later, setting out without air cover to attack the Japanese troop carriers off the Malaya coast, the new battleship HMS Prince of Wales and the old battle cruiser HMS Repulse are sunk by air attack. The one asset the Allies posse is good and fast improving Intelligence.
Intelligence Prior to Pearl Harbour.  Due to the time that it took their Embassy in Washington to decypher their instructions from Tokyo, the Japanese diplomats hand over their formal declaration of war after President Roosevelt had learnt that Pearl Harbour was being attacked. But as the US had intercepted and decrypted the message somewhat faster; they had already read it. The Japanese had changed their military codebooks a few weeks before, so both the US and British codebreakers were still slowly rebuilding their ability to read these. But a US army team led by Frank Rowlett, whose chief was the veteran code-breaker William Friedman, had in the autumn of 1940 broken the Japanese diplomatic cypher machine ‘Type B’, known to the US as the Purple machine. Friedman was wont to say that his team produced the decrypts by magic, and the name stuck. When a party of four US Codebreakers came to BP in January 1941 they brought with them a copy of this machine. BP was able to reciprocate not only by telling them about the breaking of Enigma, but by providing much of the then current codebook for the Japanese general naval code, JN-25.  The Magic decrypts in the weeks leading up to Pearl Harbour have made it abundantly clear that an attack somewhere in the South Pacific region is planned. And a large Japanese transport force has been detected and tracked by a Y station of the British Far East Combined Bureau Intelligence Centre, now in Singapore, heading south towards the Malaya peninsular; a Magic intercept has revealed that its role is to land an invasion force at Kota Bharu in NE Malaya.  But there are only a few messages directing attention to Pearl Harbour, which lay well to the East of the likely Japanese raw-material targets.  The Japanese carrier force employs strict radio silence, and cunningly their call-signs and wireless operators have been transferred to other warships that remain in Japanese home waters.  The US obtains various indications that Pearl Harbour is a target in the hours leading up to the raid, but fails to break out of peace-time routine to act fast on these.  If this can be understood, if not excused, it is more difficult to understand how General MacArthur on the Philippines, an obvious target within range of land-based Japanese aircraft, allowed his aircraft to be devastated, lined up in serried rows on their Manila airfield, on the day after Pearl Harbour.  It is sometimes suggested that the UK had more specific material from decrypts about the Japanese intention to attack Pearl Harbour but that Churchill suppressed this, to ensure that America would enter the war.  The Official History of British Intelligence in WWII chooses not to cover the war in the Pacific, but it does have a couple of paragraphs summarising the view of Whitehall on the Japanese intentions prior to Pearl Harbour. Our Intelligence authorities were quite clear that the Japanese intended to expand southwards and expected an attack on Malaya.  They implicitly excluded the prospect of a direct Japanese attack on US possessions.  The Official Historian concludes: “In the British archives there is no intelligence of any importance that was not available to the Americans”.

Breaking the Japanese Codes.  BP had considerable experience of breaking Japanese codes, for these were one of the priority fields of work for GCCS between the wars. Their first expert on Japanese codes was Ernest Hobart-Hampden. Until 1931 he was able to read much of the Japanese diplomatic traffic.  In the early 1920’s William ‘Nobby’ Clarke, a veteran of Room 40 who had stayed on when GCCS was formed in 1920, had persuaded RN officers in the Far East to intercept Japanese signals.  In 1926 Lt Eric Nave of the Royal Australian Navy, on loan to the British at the naval base at Shanghai, broke the Japanese ship movement reporting code, the Tasogore.  All the material intercepted by Eric Nave was sent back to the Admiralty and on to GCCS, who returned any further decrypts they made to Nave. Working together, by the end of 1928 the main Japanese naval operational code could be read, but then in the early 1930s the Japanese improved their cyphers. Despite this setback, GCCS continued to read much Japanese material.  In 1933 Capt John Tiltman broke the Japanese military attaché code, but in 1934 the Japanese adopted a machine for this traffic. Oliver Strachey and Hugh Foss at GCCS soon broke it, getting the Naval Signals School at Portsmouth to build an emulator for the machine. Oliver, a brother of Lytton, had been an Army code-breaker in WWI; the tall and lanky Hugh, who had grown up in Japan where his father was a missionary, was one of the great eccentrics of BP. By November 1934 they had broken the very similar Type A machine, now used for Japanese diplomatic traffic. In 1935 the Metropolitan Police signals expert, Harold Kenworthy, built for GCCS the J machine emulator for the Type A.  The Far East Combined Bureau was set up in Hong Kong in 1935 to coordinate and lead the local Intelligence work. At the end of 1937 the Japanese again improved their cyphers, adopting a super-encypherment system which used a four-digit group codebook , encyphered by adding digit by digit a ‘random’ group from a book. This method continued to be used by both the Japanese army and navy throughout the war. By the late summer of 1938 John Tiltman had broken the Military system, probably working at BP during the first period when GCCS was in the Mansion during the Munich crisis.  But now BP has had to divert much of its effort on to the German cyphers and the lead passes to the US, though BP is to continue to play a vital part in breaking many of the Japanese cyphers.

BP state at end of 1941.  At the end of 1941, BP is a very different place than it had been at the beginning of the year. Then there had been some 900 staff, now there are 1,600.  At least 6 out of every 10 of these are women. The two bombes at the beginning of the year have become 16, and the Wrens are now expert at handling them. Six bombes are installed in Hut 11 at BP, with 10 dispersed to nearby mansions at Adstock and Wavendon.  At BP the staff is still in the wooden Huts.  At the beginning of the year the only Enigma keys they were reading were the Red general Luftwaffe key and the Brown used by the German beam navigation specialists.  Now Hut 6 is also reading two further Luftwaffe keys, the Light Blue and Mustard; they finish the year by breaking Brown II.  Hut 6 is also reading, from time to time, four German army keys, two on the Eastern front, and two in North Africa.  At last Hut 8 has achieved the remarkable feat of breaking the main German Home Waters naval key, Dolphin. The team in the Cottage had broken the Italian navy Enigma, now replaced by the C38m that is also being read by BP. That ISK team in the Cottage has now broken the Abwehr Enigma key, and BP is also reading the German railways key, Rocket.  As well, BP is reading numerous hand cyphers of all three services.  Hardly surprisingly, the reputation of BP now stands very high amongst those few who know the source of the excellent Intelligence. Gradually BP is building up what will become a vast depository of information on the enemy. They have gained themselves the “extreme priority” which will stand them in good stead in all the battles that lie ahead. 

The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed.