Fortunately, the exchanges between the British codebreakers in Singapore and their American colleagues in Corregidor had doubled the number of code groups recovered. Many routine messages and stereotypical movement reports could now be read if not immediately at least within a day or two. Those parts of the JN25b codebook that had been rebuilt were still no help in deciphering detailed operational messages. Nevertheless, even routine messages when combined with traffic analysis, direction-finding and radio-fingerprinting were capable of providing enough information to build up a substantial picture of what was going on.
Two days after the Imperial Conference gave the go-ahead for war preparations, the British codebreakers in Singapore reported, probably from the Dockyard Code broken by Nave rather than from JN25, that all naval vessels of the Japanese Combined Fleet, under Admiral Yamamoto Isoruku, had been recalled to base for the annual reorganization. Since they would normally have remained at sea for two more months, the British codebreakers concluded that it seemed likely that the Combined Fleet was being prepared for war.
Meanwhile, Tojo was cranking up the screw at home, petitioning the Emperor to approve a military offensive. Konoye's determination to avoid war with America at all costs had led him to offer to meet Roosevelt for peace talks, a move seen within the army as a loss of face for Japan. With Konoye still arguing that the negotiations with America be allowed to run their course, Tojo let it be known that the army would withdraw its support for his government if he did not back war. No Japanese Government could hope to survive without the army's support. Konoye was now dead in the water. He resigned on 18 October and Tojo took over as Prime Minister while retaining the War Minister portfolio. The outbreak of war with Britain and America was simply a matter of time.
Bletchley Park now deciphered a circular to all Japanese consular posts abroad that ordered them to provide ‘prompt and detailed reports of the movements of British, American, French and Dutch warships, aircraft and troops’.
The Far East Combined Bureau began picking up almost daily messages from the Japanese Consul-General in Singapore relating to subversion and intelligence gathering. He reported on the precise strength and location of defensive forces and anti-aircraft guns in the area around Singapore itself together with full estimates of the time it would take to knock them out. He described RAF organization and tactics; dispatched ‘fishermen with a knowledge of surf conditions on the east coast of Malaya’ to Bangkok to brief the Japanese Military Attaché, and reported that members of the anti-British Young Men's Malayan League KAME would act as guides for Japanese troops if they invaded Malaya.
Oshima was gung-ho at the prospect of a full-blown Axis military alliance against Britain. The successful attack on the Soviet Union suggested the Wehrmacht would have no problem occupying the United Kingdom, he reasoned. Japan should synchronize an attack on Malaya with the launch of the German invasion force across the English Channel, a move he believed would provide an answer to those concerned over the dangers of becoming embroiled in a conflict with America. ‘Germany is not considering any compromise with Britain and is sure that after the occupation of the British Isles – even if an attempt is made to carry on the war from a dominion with American help – the British Empire will break up, after which a compromise peace with the Americans can be easily arranged.’
By the end of October the naval intercepts had left no doubt that the Combined Fleet had been mobilized. Given the strict Japanese attention to radio security, traffic analysis backed by direction-finding and radio-fingerprinting still remained the codebreakers’ most effective means of tracking the movement of the Imperial Navy. By early November, although Singapore was still locating the bulk of the Combined Fleet in its home port of Kure, it had begun to detect large movements of Japanese transport ships south.
‘There are increasing indications that Japan is preparing for further advances southwards with… the Dutch East Indies, Siam, Malaya or Burma, or any or all, as her objectives,’ wrote Malcolm Kennedy in his diary. ‘As she is feeling the pinch of economic strangulation seriously now, as a result of the freezing orders and of her inability to obtain oil, it should cause no great surprise if she had a crack at the Dutch East Indies before long.’
A firm indication that one of the key targets was actually Malaya came a few days later. The Far East Combined Bureau deciphered a message from Tokyo to the Consul-General in Singapore informing him that the Asamu maru, which was leaving in just over a week's time, would be the last Japanese ship to call there. An RAF ‘Y’ interception unit, 52 Wireless Unit, arrived in Singapore in early November to reinforce the British codebreakers, but resources remained stretched.
Meanwhile, the Japanese preparations for war escalated. Yamamoto formally concluded a ‘Central Agreement’ with General Count Terauchi Hisaichi, Southern Army Commander, setting out Japanese plans to turn the whole of South-east Asia into a Japanese-controlled Greater East Asia Co-Prosperity Sphere.
None of these plans were available to the codebreakers at Singapore, Corregidor and Pearl Harbor. But despite the problems with JN25, the preparations for the initial phase of the Japanese offensive were clearly reflected in the naval traffic. Over the next two weeks, the codebreakers continued to record the movement of elements of the Combined Fleet south towards Hainan.
The Purple messages remained the best source of detailed intelligence on Japanese intentions. On 19 November the gaimushoo warned the Japanese Embassy in London that the international situation was ‘tense’ and told it to await a coded weather message on Japanese overseas radio that would indicate the opening of conflict with Britain, America or Russia. ‘With America, the words: higashi no kaze, ame [Easterly wind, rain]. With Soviet, the words: kita no kaze, kumori [Northerly wind, cloudy]. With Britain, including invasion of Thailand, the words: nishi no kaze, hare [Westerly wind, fine]. On receipt of these code words all confidential books are to be burned.’
On the same day, Yamamoto sent a message to all his flagships ordering them to test ‘the communications set-up required upon opening hostilities’. It was followed a day later by a second Yamamoto message to Fleet Commanders informing them that ‘the second phase of preparations for opening hostilities’ would begin at midnight that night. Both these messages from Yamamoto were intercepted by the British and the Americans, but the limited reconstruction of the JN25b codebook did not allow either to be deciphered.
Nevertheless, November provided the codebreakers with their most productive period so far on JN25b, recalled Neil Barham, one of the leading codebreakers at the Far East Combined Bureau. By now they had recovered more than 3,000 code groups, a tenth of the book, and although the recoveries they had made so far were still largely confined to routine messages, the General Operational Code produced ‘intelligence covering a wide field’.
The Singapore codebreakers had worked out from a mixture of traffic analysis and codebreaking that the Japanese aircraft carriers had been reorganized into five squadrons. They had also deduced that a large number of passenger liners had been incorporated into the Combined Fleet as auxiliary cruisers, although in London ‘this was not regarded as entirely reliable or authentic’.
The American ‘Hypo’ communication intelligence research unit at Pearl Harbor was not working on the JN25 code and as a result had become highly skilled at traffic analysis. On 26 November it produced detailed evidence of the formation of a special task force consisting of 4 Kongo-class battleships, 4 aircraft carriers, 2 seaplane carriers, 8 heavy cruisers, 13 light cruisers, 56 destroyers and 19 submarines. The task force was believed to be moving south. Singapore confirmed that ‘a special force has been organized for an operation’ and that large numbers of Japanese vessels of varying sizes were now known to be moving southward.
Despite the heavy movement of Japanese vessels, there had been a drop in the amount of Japanese radio traffic as Yamamoto ordered all units to maintain radio silence. Among those messages which were intercepted but could not be deciphered were
two speaking of plans for ‘exhaustive conscription’ and the need to prepare extensive medical facilities. There was also a signal from Admiral Prince Fushimi Hiroyasu, the retired Chief of the General Staff, to his successor Admiral Nagano Osami, in which he said: ‘I pray for your long and lasting battle fortunes.’
The most intriguing, with hindsight, were a report from Imperial Headquarters on the identity of any ships operating in the North Pacific and another that spoke of a ‘northern force’ as well as the southern task force that was being monitored. Had either of these last two been deciphered, they might have pointed to the fact that there was an additional target apart from Malaya, the Philippines and the East Indies. But they were not.
While the codebreakers remained completely unaware of the Japanese plans to attack Pearl Harbor, there could be little doubt they were about to go to war. Meanwhile, the Japanese team continued to negotiate in Washington as if nothing was happening. But the Purple messages made it clear that Tojo did not expect, or indeed want, the talks to succeed and that once they failed ‘things are automatically going to happen’.
Fully aware of the Japanese preparations for war, Cordell Hull, the US Secretary of State, presented the Japanese negotiating team with an ultimatum. Japan must withdraw not just from Indochina but from China. No-one in the Washington administration could have seriously believed that the Japanese would accept these terms.
When the US demands reached Tokyo, both the army and the navy were furious. It was clear once again, as it had been during the drafting of the 1921 Washington Naval Treaty, that the Anglo-Saxon nations did not regard Japan as an equal and were determined to keep her down. Now at last Oshima was told what was happening. ‘Breakdown of the Washington talks is inevitable,’ the gaimushoo said in a signal sent to Berlin on 30 November and deciphered at Bletchley Park. Oshima was to brief Hitler immediately. ‘The British and American attitude has been provocative and they have continued to move troops in all parts of East Asia,’ the message said. ‘Japan is therefore compelled to make corresponding movements and armed collision with British and American forces is feared. This may happen sooner than expected.’ A day later, Japanese diplomats in London were ordered to destroy their codes and ciphers. They were issued with a limited number of special codewords to receive their remaining messages from Tokyo and began making preparations to leave. It was not until the next day that Washington received a similar series of messages, Rowlett recalled. ‘As I look back at all the messages and other information available to us at that time regarding the Japanese intentions, it becomes crystal clear to me that this message ordering the destruction of certain of Washington's codes provided the necessary evidence that the Japanese unquestionably intended to take some action which would make war between the United States and Japan a certainty.’
The Purple messages from Tokyo to Berlin also revealed the Japanese anxiety to ensure that Germany would support it in any confrontation with America. Oshima's problems in confirming this beyond a shadow of a doubt were exacerbated by Hitler's absence at Rastenberg, controlling events on the Eastern Front. The Japanese Ambassador managed, however, to secure a firm promise from Ribbentrop. ‘Should Japan become engaged in a war against the United States, Germany, of course, would join the war immediately,’ the German Foreign Minister said. ‘There is absolutely no possibility of Germany's entering into a separate peace with the United States under such circumstances. The Führer is determined on that point.’
Meanwhile, the Allied intercept operators stationed around the Far East were reporting that the Japanese Navy had changed all of its call signs. Since these would normally remain in use for six months and had only been changed three weeks previously this was yet more evidence that a Japanese offensive was imminent.
By now more detailed analysis of the deciphered messages and radio traffic associated with the Japanese ‘special task force’ had allowed the British codebreakers to name most of the vessels involved and to state that it had been put together ‘to carry out an operation in the south. Indicators are an attack on Siam, including possibly a landing on the Kra Isthmus [the narrow strip of land north of the Thai–Malay border].’
A diplomatic telegram from the Japanese Minister in Bangkok to Tokyo revealed that the Thai Cabinet was hoping Britain could be made to strike the first blow at Thailand to give Japan an opportunity to intervene. Landings at Singora and Patani, the most suitable places along the eastern Thai coast, and at Kota Bharu, the most northerly town on Malaya's north coast, would induce the British to attack the Japanese forces through Thailand, which would then declare war on the British ‘aggressors’.
The British had accurately predicted the Japanese landing sites five years earlier and had devised plans for Operation Matador, under which British and Indian troops would occupy the Kra Isthmus to oppose the landings. But with continuing uncertainty over the American position and its forces stretched by the war with Germany, Britain could not afford to make a pre-emptive strike. The authorities in Malaya were ordered to hold the start of Operation Matador until the American position became clear.
Churchill sent a naval task force to the Far East to provide a deterrent against Japanese attack. Force Z arrived in Singapore on 2 December to a tumultuous welcome. It was led by the brand-new battleship HMS Prince of Wales and the battle-cruiser HMS Repulse. It was also due to comprise the aircraft carrier HMS Indomitable and four destroyers: HMS Electra, HMS Express, HMS Tenedos and HMAS Vampire. But Indomitable had run aground in the West Indies, leaving Force Z with no air support of its own and dependent on the RAF's Malaya-based Brewster Buffaloes, which had a very limited operational range.
The main question now was when precisely would the attack take place? The codebreakers guessed that the departure of the Japanese task force from Hainan would be preceded by a change in the JN25 additive. It came on 4 December with the introduction of a new table. While this robbed them temporarily of their limited ability to break the naval messages, it was not as bad as it might have been had the codebook itself changed. They now had nearly 4,000 code groups recovered, allowing them to get out additive on many of the most common messages and make further inroads into the codebook itself.
Meanwhile, Oshima was making frantic efforts to firm up Hitler's promise of support against America. The Japanese Ambassador told Tokyo he had negotiated a ‘secret agreement’ with Germany and Italy, adding that ‘should a state of war arise between Japan and the United States, Germany and Italy for their part will consider themselves at war with the United States’.
The British codebreakers at Bletchley Park, Singapore and Hong Kong were now on full alert, waiting for the ‘Winds’ code message. ‘Owing to the critical situation was on duty at the office from 9 a.m. yesterday until 9 p.m. today,’ noted Malcolm Kennedy in his diary for 6 December.
From now on, for the time being, we are to take turns about at night in case anything calling for immediate action comes in.
Incidentally, the All Highest [Churchill] is all over himself at the moment for latest information and indications re Japan's intentions and rings up at all hours of day and night, except for the four hours in each 24 [2 p.m. to 6 p.m.] when he sleeps. For a man of his age, he has the most amazing vitality. His chief form of recreation, I gather, is to get out onto the Admiralty roof whenever there is a raid on London and shake his fist at the raiders! As there have been no serious raids on London now for six or seven months he must be missing his recreation.
It was late on Sunday 7 December local time when Hong Kong reported having heard the coded Winds message. It had said: ‘higashi no kaze, ame; nishi no kaze, hare [Easterly wind, rain; Westerly wind, fine]. This was the signal that Japan was about to declare war on both Britain and America.
A few hours later, in the early hours of Monday 8 December, Singapore time, the first Japanese troops began landing on Kota Bharu beach in northern Malaya. It was the first in a carefully coordinated series of attacks against Malaya, the Philippines, Hong Kong and
the only real surprise, Pearl Harbor.*
The inability to decipher any detailed operational orders sent in the JN25 naval code had prevented the codebreakers from noting the existence of the ‘northern force’ that was to attack Pearl Harbor. Persistent references to training by the Japanese 1st Air Fleet in the use of torpedoes in shallow water were also intercepted but not deciphered as a result of the difficulties with JN25. These messages would also have pointed to an attack on Pearl Harbor, where the US ships moored in Battleship Row were protected by waters too shallow for conventional torpedoes.
The surprise air attack on the US Navy's Pacific Fleet on the morning of Sunday, 7 December 1941, left 18 ships, including 7 battleships, destroyed or badly damaged, 164 aircraft destroyed and 2,341 US servicemen dead. Roosevelt denounced it as ‘infamy’ and promptly declared war on Japan.
That the attack on Pearl Harbor had come as much as a shock to the British codebreakers as to their American counterparts is shown by Malcolm Kennedy's diary entry for 7 December. ‘A message received just before leaving the office this evening had indicated that the outbreak of war was probably only a matter of hours. But the news on the 9 p.m. wireless that Japan had opened hostilities with an air raid on Pearl Harbor more than 3,000 miles out in the Pacific came as a complete surprise.’ Kennedy's incredulity remained undiminished the next day. ‘That Japan, if she struck, would strike swiftly and heavily was only to be expected but the widespread nature of the attacks has come as a surprise and shows extraordinary audacity,’ he wrote.
*Although the attack on Pearl Harbor took place on Sunday 7 December, the relative location of Hawaii and Malaya on either side of the international dateline meant that in ‘real time’ the attack on Malaya occurred first.
The Emperor's Codes Page 12