Although the joint British and American efforts against JN25 did not allow them to break detailed operational messages, they were able to track the Imperial Japanese Navy and to build up a good picture of its activities through a combination of those messages they could decode and traffic analysis. The latter technique had been greatly helped by improvements in the British direction-finding network and by the introduction in early 1941 of radio-fingerprinting. This was a method of identifying transmitters by recording their individual idiosyncrasies on film which ensured that once a transmitter was identified to a ship or location, it could always be recognized, whatever call sign it used.
Despite America's new-found enthusiasm for intelligence cooperation with the British against Japan, it continued throughout the latter part of 1940 and well into 1941 to show remarkably little interest in supporting Britain's attempts to rein in Tokyo's expansionist ambitions in China and South-east Asia. But as the deciphered Purple messages began to establish a clearer picture of Japan's real intentions, the American attitude was to change dramatically.
The Japanese signalled their intentions from the very start of 1941, informing all their missions abroad that all future intelligence and propaganda must be directed at aiding the expansion of Japanese territories southward ‘in order to secure supplies of war commodities’. Attempts to undermine and subvert the position of the British, American, Dutch and French authorities in South-east Asia and the gathering of military and naval intelligence were to be intensified ‘so that the new order in Greater East Asia may soon reach fruition’.
Nevertheless, there was a good deal of disagreement within the Japanese Government of Prince Konoye Fumimaro over the extent to which it should go to achieve regional dominance. The Imperial Japanese Army, War Minister General Tojo Hideki and Foreign Minister Matsuoka Yosuke were quite prepared to provoke war with America and Britain but the navy was far more cautious. Admiral Yamamoto Isoruku, Commander of the Japanese Combined Fleet, had warned Konoye on the signing of the Tripartite Pact that Japan should at all costs avoid a conflict with the United States. The initial element of surprise and Japan's better preparedness for war would allow it to make early gains over the short term but it could not hope to win a sustained conflict against US economic might.
As a result of this conflict within the Japanese Government, a series of mixed messages emerged from Tokyo during the early part of 1941. The British codebreakers intercepted one telegram from the Japanese Military Attaché in Stockholm which spoke of the possibility of using the predicted German invasion of Britain as an opportunity to occupy French Indochina, but insisted that Japan had no wish to be drawn into a fresh war in the region.
The Bletchley Park report on the contents of the intercepted message was heavily disguised to protect its source:
We have learned through a very reliable channel that the Japanese Military Attaché in Stockholm has recently – on or about 24 February – made remarks to the following effect in the course of an intimate conversation.
The German General Staff has assured the Japanese General Staff that the United Kingdom will be invaded this spring. The Japanese General Staff have hitherto believed this but have recently begun to have doubts. As a result the Japanese Military Attachés in Europe have been instructed immediately to investigate the possibility of a German invasion of the UK as Japan wishes to co-ordinate her actions in the Far East with such an invasion. Japan does not wish to be drawn into a new war in the Far East but hopes to exploit conditions in Europe with a view to occupying French Indochina. If Japan receives information that Germany is not, or not yet, in a position to undertake a direct attack on the UK she will proceed with greater caution in regard to French Indochina.
Very few people outside of Bletchley Park were aware at this time of the work going on there and of the origins of some of the remarkable intelligence they were producing. As a result the ‘Most Secret Source’ reports were often treated with extreme scepticism, even by many British intelligence officers. The disguised nature of this particular report led one disbelieving member of MI2, the department of military intelligence covering the Far East, to say that if it were true ‘the Japanese Military Attaché in Stockholm is indiscreet to the point of imbecility’.
Despite the suggestion that Tokyo was not interested in being drawn into a fresh war, Japanese officials in London were ordered to restrict their dealings with the British and to be ready to leave for home at a moment's notice.
Purple messages passing between Oshima in Berlin and the gaimushoo in Tokyo also provided the codebreakers with some of the earliest evidence that Hitler was about to turn on Stalin, his ally of convenience in the early days of the European war. They revealed in February 1941 that Matsuoka would be travelling to Europe, via Moscow, to have talks in Rome and Berlin with both Mussolini and Hitler. The Japanese Foreign Minister was reticent in his messages back to Japan, but he revealed that Hitler had hinted that he was preparing to invade Russia.
Although Matsuoka was in the process of negotiating a neutrality treaty with the Soviet Union, the Japanese Foreign Minister told Hitler that ‘it would be practically impossible to foretell what attitude the government of Japan would take at the time of such an eventuality’. But he added that in his opinion, he ‘could not imagine that Japan would not attack the Soviet Union, via Manchukuo, if war were declared between Germany and the Soviet Union’.
The Purple intercepts also revealed Matsuoka's request for Japanese access to ‘all Germany's inventions and lessons from the war’. It was immediately met, with Oshima reporting that four representatives of the Junkers aircraft company were travelling to Tokyo to set up a joint military aircraft factory, and adding that Germany was willing to loan Japan a number of ships. Oshima was told to inform the German Government that in return Japan was prepared to send it rubber acquired from Thailand.
The Führer's hints of the forthcoming German attack on Russia were confirmed in April, again through the Purple messages passing between Oshima and Tokyo. One US Army codebreaker who worked on the key message – broken in both London and Washington – said it described a meeting between the Japanese Ambassador and Hermann Goering, Hitler's most senior lieutenant. ‘Goering was outlining to Oshima Germany's plan to attack Russia, giving the number of planes and numbers and types of divisions to be used for this drive… I was too excited to sleep that night. It was the liveliest news for many a day. We and the British informed the Russians about it but they were too dumbfounded to believe it at first.’
In fact, the British codebreakers working on the Nazi Enigma ciphers had been reporting German preparations for Operation Barbarossa for some time. But the warnings to Stalin did not come until later. Even the meeting between Goering and Oshima had failed to convince many in Whitehall that Hitler was about to turn on his Russian allies. The Germans were simply trying to intimidate Moscow, British military intelligence argued.
The codebreakers were not believed until 10 June, twelve days before the invasion, when the Japanese diplomatic section in Elmers School translated two messages from Oshima. One reported that Hitler had told him personally that war with the Soviet Union was now inevitable. The other suggested to his bosses in Tokyo that ‘for the time being I think it would be a good idea for you, in some inconspicuous manner, to postpone the departure of Japanese citizens for Europe via Siberia. You will understand why’.
Two days after Operation Barbarossa began, Oshima reported that Ribbentrop had asked him for as much help as Tokyo could provide. The Japanese Ambassador described the German successes on the Eastern Front in glowing terms. More than 2,000 Soviet aircraft had been destroyed already, he said: ‘Thus the Soviet air forces were completely annihilated and the German Air Force has gained, already, mastery of the air.’
Matsuoka's response seemed to offer the Germans what they, and Oshima, wanted. ‘Japan is preparing for all possible eventualities as regards the USSR in order to join forces with Germany in actively combating the communi
st menace,’ the Japanese Foreign Minister promised.
But although extending Japanese influence by attacking north into the Soviet Far East was still regarded as a feasible option even by some elements within the army, the predominant view was that Japan should strike towards the south, with an attack on Malaya through Thailand seen as the most likely starting point. One Cable and Wireless ‘drop copy’ supplied to the FECB in late April 1941 was an ominous message to the Consul-General in Singapore from the Japanese Military Attaché in Bangkok, asking for regular weather reports and the names of ‘suitable residents’ who could be used as observers.
Confirmation that the favoured expansion was southwards came with a telegram resulting from an Imperial Conference held on 2 July 1941. An Imperial Conference was a plenary meeting of the Japanese Government called only when decisions of great moment were to be taken and held in the presence of the Emperor himself. Hirohito looked down on the proceedings from a raised throne in front of a gold screen. Prince Konoye and his ministers were suitably placed below the imperial presence, seated around a rectangular brocade-covered table.
The gaimushoo telegram to Oshima informing him of the conference decision, which was deciphered in Bletchley Park's Japanese diplomatic section on 4 July, made it clear that Japan was intent on expanding its empire into South-east Asia. The first step was the occupation of the whole of French Indochina, by force if necessary, to provide bases that would allow it to launch attacks against Malaya and the Dutch East Indies. Should Britain or America attempt to interfere, the Japanese would ‘brush such interference aside’, Oshima was told.
The Japanese intentions did not remain secret for long. ‘Judging from confidential information and newspaper reports alike, Japan appears to have given up the idea of joining in against the Soviets – anyway for the time being,’ Malcolm Kennedy noted in his diary a few days later. The Japanese were ‘preparing for a showdown in Indochina, the idea apparently being to obtain naval and air bases so as to be in a position to strike at Malaya later’.
As alarm bells began to sound around Whitehall's corridors and across the Atlantic in Washington, the British Government ensured that the full import of what the Japanese were doing was made clear. ‘There seems to have been a pretty good “leakage” to The Times about Japan's intentions to occupy bases in Indochina,’ Kennedy noted. ‘In order to prepare our own people for the coming shock it has apparently been thought advisable by HMG to indicate to the press what is about to happen.’
On 24 July Bletchley Park read a Purple message confirming that Vichy France had agreed to allow the Japanese to occupy southern Indochina, ostensibly to protect it from a possible British attack, a suggestion that infuriated Kennedy. ‘Anything more contemptible than this attempt on the part of Vichy to hide up its own supineness is difficult to conceive,’ he wrote in his diary.
The Americans were in the process of negotiating with the Japanese in an attempt to find a peaceful end to the tension building up in South-east Asia. As a result, there had been a dramatic increase in the number of messages between the Japanese Embassy in Washington and Tokyo, all of which were read by the US code-breakers. The perfidious Japanese attitude evident in the messages and Tokyo's privately dismissive attitude to the talks left US officials furious. Under pressure from the powerful China lobby in America, Roosevelt finally agreed to take firm action, imposing large-scale trade sanctions against Japan in tandem with the British and the Dutch. At a stroke, Japan's foreign trade was reduced by three-quarters and its oil supplies by 90 per cent.
The British, who had earlier been so frustrated by the Americans’ lack of action, now became concerned at the extent to which Washington was prepared to go. The BJ (blue jacket) decrypts showed that Japan was already ordering businessmen and non-vital consular and diplomatic officials home from various points in the British Empire. The Foreign Office feared that the Japanese would now be forced to invade the British and Dutch Far East colonies simply to obtain the raw materials upon which the Japanese economy depended, and that war was therefore inevitable.
8
EAST AND WEST WINDS
The Purple decrypts of Oshima's reports to Tokyo were now providing the Allies with a good deal of useful intelligence on the war in Europe. The Japanese Ambassador had unique access to the thinking of the High Command and even of the Führer himself. As Hitler sought to draw the Japanese into the war with the Soviet Union, he had Oshima flown to his Eastern Front field headquarters in Rastenberg, East Prussia, for private briefings on the progress of Operation Barbarossa. By August, the Japanese Ambassador's telegrams to Tokyo were speaking of staggering Soviet losses ‘estimated at between five and six million’.
The Purple decrypts also revealed the extent of the pressure exerted by Hitler and his lieutenants to get Japan to declare war on the Allies. Oshima reported that ‘the Führer was not at all satisfied with Tokyo's attitude, particularly with regard to the continuation of Japanese–US negotiations’. This campaign culminated, in mid-August, in Hitler promising that ‘in the event of a collision between Japan and the United States, Germany would at once open hostilities with America’.
Winston Churchill had just returned from a conference in Newfoundland with President Roosevelt at which they had cemented their special relationship and agreed jointly to warn the Japanese against any aggression towards Malaya or the Dutch East Indies. The British Prime Minister received all the most important Bletchley Park decrypts each day, delivered in a yellowing leather box by Colonel Stewart Menzies, the head of MI6. When Churchill read the Oshima report of Hitler's promise to attack America, he sent it back to Menzies immediately. ‘In view of the fact that the Americans themselves gave us the key to the Japanese messages it seems probable the President knows this already,’ he wrote beneath the decrypt. ‘But anyhow it is desirable he did know it. WSC.’
Two weeks later, on 6 September, Churchill went to Bletchley Park himself to thank the codebreakers for the unprecedented flow of intelligence he was receiving, both on the war in Europe and the increasingly tense situation in the Far East.
‘Winston made us a very nice speech of thanks and went off to lunch at Blenheim,’ noted Nobby Clarke. ‘He had arrived in the usual cortège of cars with flags flying and must have been spotted by the local inhabitants. For lunch, the BP staff crowded out into the town and no doubt talked about the visit to all and sundry. Late that afternoon, an order came round to say that his visit must be kept secret.’
Earlier that day, at an Imperial Conference in Tokyo, the Japanese Government had been told by Tojo, in the bluntest terms possible, that there were only three alternatives open to them. They could back down in the face of American pressure and withdraw from Indochina, and eventually from China; they could stall for more time and continue negotiating with the Americans; or they could prepare for war.
The wind was taken out of the Japanese War Minister's sails by the Emperor, who reminded those present of the awesome consequences of what they were discussing. Tojo had hoped to railroad the Japanese Cabinet into authorizing an attack on Malaya and the Philippines, effectively declaring war on Britain and America. In the end, he was forced to agree that Japan should continue to negotiate for peace. But in return for this temporary compromise, he secured the Emperor's sanction to prepare for war.
This was to be only a minor setback for Tojo, but it was confusing for those attempting to interpret the Purple intercepts. Although the messages between Berlin and Tokyo were full of indications of the Japanese preparations for war with Britain and America, they lacked any conclusive evidence that the threatened confrontation was imminent. With Oshima under pressure from Hitler and Ribbentrop to persuade his political masters to open a second front against Stalin, he was as irritated by the lack of information as the code-breakers.
‘It is an indispensable condition that envoys serving abroad should be kept apprised of the Government's policy,’ the Japanese Ambassador in Berlin complained to Tokyo. ‘One can only assu
me that the lack of clear instructions from you in these matters is from considerations of secrecy or because no definite policy has yet been formulated.’
The difficulties the codebreakers faced in ‘rebuilding’ the new JN25 codebook meant that the evidence from Japanese naval traffic was sketchy. Although Tiltman had broken JN25, he was now more concerned with Japanese military codes, and Bletchley Park's small Japanese section was limited in its ability to assist. Nor were the US Navy codebreakers at OP-20-G in Washington any help, having placed all their emphasis on diplomatic ciphers. During a visit to Washington in the late summer of 1941, Denniston was unimpressed with the priorities of the American codebreakers who had failed to read a single JN25b message. ‘They have done very well, but only in one thing – Japanese diplomatic. Japanese naval and Japanese military are still behind,’ he said. The veteran British codebreaker was particularly critical of OP-20-G's allocation of resources.
I feel they have really neglected the naval work. The staff there is interested in the solution of the cryptanalytical problem and the contents of the results do not concern them. The finished document based on the translation of the cypher telegram is not always of the standard we aim to maintain. It might nearly be said that they wish to find mathematical formulae or mechanical methods to solve their problems while we aim to provide the various intelligence services with a clear and accurate text.
Denniston persuaded OP-20-G that since America was supposed to be taking the lead on Japanese codes and ciphers, leaving Bletchley Park to concentrate its resources on the German Enigma machine, it should do more work to help the Far East outposts in Singapore, Corregidor and Hawaii on Japanese naval material and in particular JN25. ‘The section dealing with Japanese diplomatic is well staffed by cryptanalysts and translators and has a priority claim on the Hollerith machine room. This section has undertaken collaboration with BP and Singapore in investigation of the Japanese naval ciphers and they now regard this, as we do, as one of their most important research jobs.’
The Emperor's Codes Page 11