MAGIC reveals Japan’s plans

On 4 July 1941, Independence Day, Secretary of State Cordell Hull, at the request of President Franklin Delano Roosevelt, sent a telegram to the embassy in Tokyo. Ambassador Joseph Grew was to tell the Japanese prime minister, Prince Fumimaro Konoe, that reports were reaching Washington “from various sources” of Japan’s intention to enter the war against the Soviet Union, and that the American government asked for an assurance that they were untrue. That same day the Japanese ambassador in Washington, Admiral Kichisaburō Nomura, delivered a letter for Hull at the State Department: the foreign minister had authorised him to give an assurance that there was no disagreement within the Japanese government over its intention to settle relations with the United States “on a fair basis”. Both sides spoke the language of peace, but the Americans knew more about Japanese intentions than the Japanese suspected. Since the autumn of 1940 their cryptologists had been reading the machine cipher of the Japanese Foreign Ministry, and the decrypted cables, known in Washington as “Magic”, showed in July 1941 that Tokyo had decided not to attack the USSR for the time being, but to move south, first into southern French Indochina.

It is often said that on this day American intelligence, thanks to intercepted MAGIC cables, “discovered Japanese plans to attack South-East Asia”. This needs two qualifications. First, at the beginning of July 1941 Japan had not yet decided to attack South-East Asia. The Imperial Conference of 2 July resolved to occupy southern Indochina, with the consent of Vichy France if possible, and to prepare for a further advance southwards, “not shrinking” from war with Britain and the United States. The decision for war was taken only in the autumn. Second, Washington’s knowledge did not arrive in a single day. It was built up from many cables intercepted and decrypted during July, and the most important of them, a circular summarising the decisions of the conference, sent from Tokyo on 2 July, was translated only in August, according to the documents published by the congressional committee that investigated the attack on Pearl Harbor. 4 July is therefore a conventional date for the moment when the Americans began to know which way Japan would turn.

Purple and “Magic”

The Japanese Foreign Ministry enciphered its most important correspondence with its embassies using an electromechanical machine that the Japanese called the “Type B cipher machine” (colloquially “Type 97”) and the Americans called “Purple”. The name was a play on colours: American cryptologists had called an earlier, simpler cipher system “Red”, and purple is made from red and blue. The machine was introduced in 1939. The work of breaking it was done by the Signal Intelligence Service, the US Army’s cryptologic service, headed by William Friedman, with Frank Rowlett leading the team studying the new cipher. The cryptologists never saw the Japanese device. From hundreds of intercepted cables, from the mistakes of Japanese cipher clerks and from texts that embassies sent in parallel in an older cipher that had already been broken, they reconstructed the principle on which it worked. In September 1940 Genevieve Grotjan spotted a pattern in the ciphertexts that made it possible to reconstruct the most difficult part of the mechanism, and at the end of the month the first complete messages were read. A copy of the Japanese machine built in Washington, assembled partly from telephone exchange components, thereafter deciphered Tokyo’s correspondence with its embassies in Washington, Berlin, Rome, Moscow, Vichy and dozens of other capitals.

Further copies went to the US Navy and to the Philippines, and in January 1941 American cryptologists brought one to Britain, in return learning about British work at Bletchley Park. The cryptologic services of the army and the navy took turns decrypting the cables. They were translated by the few officers who knew Japanese, and couriers showed the folders of translations to a narrow circle: the president, the secretaries of state, war and the navy, and the chiefs of staff and of intelligence. The recipient read the text in the presence of an officer and handed it back; no copies were left. The impression made by reading a foreign government’s most secret correspondence, sometimes faster than it reached its addressee, meant that in Washington all this material was simply called “Magic”.

The Japanese considered Purple unbreakable. In April 1941 a German diplomat in Washington, Hans Thomsen, reported to Berlin that an “absolutely reliable source” claimed the Americans were reading Japanese diplomatic correspondence. The warning reached Tokyo, and Nomura himself reported similar rumours, but the Japanese ministry did not believe a machine cipher could be broken and did not replace it. In July 1941, therefore, the Americans were following Japanese policy almost in real time.

North or south

The German attack on the Soviet Union on 22 June 1941 took Tokyo by surprise and confronted Japan’s leaders with a choice they had argued over for years. Foreign Minister Yōsuke Matsuoka, who barely two months earlier had signed a neutrality pact in Moscow, now demanded a swift strike into Siberia alongside Germany. Part of the army general staff saw the German–Soviet war as an opportunity to settle accounts with its age-old rival in the north. The navy and many army officers, however, believed that the real resources – oil, rubber, tin and rice – lay in the south, in the Dutch East Indies, Malaya and Indochina.

The plan to move into southern Indochina had been maturing since the spring. After the entry into Tonkin in September 1940 and the Treaty of Tokyo, by which in May 1941 Japan had imposed peace on France and Thailand, the military wanted air and naval bases in the south of the colony, from which they could threaten Singapore and the Dutch East Indies. When the trade talks with the Dutch in Batavia broke down in June, the army and navy staffs agreed that the right to station troops in southern Indochina had to be obtained. On 25 June a liaison conference of the government and Imperial General Headquarters adopted a document on “accelerating the southern policy”, and Emperor Hirohito approved it that same day. Matsuoka, absorbed in his plan to attack the USSR, demanded that the southern move be postponed, warning that it might lead to war with Britain and the United States. He was outvoted.

On 2 July 1941, in the Imperial Palace and in the presence of the emperor, a conference adopted the “Outline of National Policies of the Empire in View of the Changing Situation”. The document stated that Japan, whatever developments in the world situation, would build the Greater East Asia Co-Prosperity Sphere, strive to end the war in China and advance southwards to secure “the foundations for self-existence and self-defence”. It was to carry out the earlier decisions concerning Indochina and Thailand, and to that end prepare for war with Britain and the United States and “not shrink” from it. As regards the German–Soviet war, Japan was not to intervene for the time being, but to make secret military preparations and, should the war develop in its favour, “settle the northern question” by force. America’s entry into the war was to be prevented by every diplomatic means, and if it nevertheless came about, Japan was to act in accordance with the Tripartite Pact, while deciding for itself when and how to use force.

It was a compromise between the advocates of the northern and southern advances, but a compromise that meant immediate action in only one direction. In the north the army began a large-scale mobilisation on 7 July, disguised as the “Kwantung Army Special Manoeuvres”, and waited for the Red Army to collapse. In the south preparations for the occupation of southern Indochina began at once. On 5 July the 25th Army, which was to move in there, was formed.

What Washington read

From the first days of the German–Soviet war Roosevelt knew that a dispute was under way in Tokyo. On 1 July he wrote to Secretary of the Interior Harold Ickes that the Japanese had been fighting among themselves for a week, trying to decide which way to jump: whether to attack Russia, or to move on the South Seas and thereby throw in their lot definitively with Germany, or to sit on the fence and draw closer to the United States. In the following weeks the intercepted cables gradually supplied the answer.

As early as 2 July Matsuoka informed the German foreign minister, Joachim von Ribbentrop, through the ambassador in Berlin, Hiroshi Ōshima, that Japan would remain faithful to the spirit of the Tripartite Pact but could not now break off relations with the USSR or weaken its efforts in the south. As its contribution to the common cause it would acquire bases in French Indochina in order to increase the pressure on Britain and the United States. That same day Tokyo sent its embassies in Berlin, Washington and Moscow circular No. 1390, summarising the decisions of the Imperial Conference: Japan would build its co-prosperity sphere “no matter how the world situation may change”, would strengthen its preparations for the southward advance and carry out “the policy already decided upon” towards Indochina and Thailand, and, if necessary, “would act in accordance with the Tripartite Pact and decide for itself when and how to use force”. The copy of the American translation of this circular published after the war by the congressional committee bears the date 8 August. But the meaning of these decisions was evident earlier from many other cables.

Other cables moved faster. At the beginning of July Tokyo concluded that Ribbentrop would not induce the Vichy government to make concessions, and asked him not to raise the matter: Japan would open negotiations itself and, if refused, would fight. On 8 July Matsuoka instructed the ambassador in Vichy, Sotomatsu Katō, to talk to the French “confidently and firmly” about an agreement on the joint defence of Indochina. On 12 July Tokyo sent Vichy terms that amounted in substance to an ultimatum. Japan demanded the right to send into southern Indochina “the necessary number” of troops, warships and air units; eight airfields, among them at Saigon, Biên Hòa, Nha Trang, Tourane (Da Nang), Sóc Trăng, Phnom Penh and Siem Reap; naval bases at Saigon and in Cam Ranh Bay; freedom of movement for its troops; and the right to issue currency for them. The reply was to come by 20 July. Matsuoka justified this by the “encirclement” of Japan by the Americans and the British, and frightened the French with the fate of Syria, which British and Free French forces were just then occupying: the only way to save Indochina for France was to place “absolute trust” in Japan.

The most telling was a cable from the consulate in Canton of 14 July, decrypted in Washington five days later. In it the consul passed on what local officers had told the attaché: “The immediate purpose of occupying French Indochina will be to achieve our aims there. Its second purpose is, when the international situation is favourable, to launch a swift attack from there. […] After the occupation of French Indochina, the next item in our plan is to send an ultimatum to the Netherlands Indies. In the capture of Singapore the navy will play the principal part. The army will need only one division to capture Singapore and only two to capture the Netherlands Indies.” The cable also announced that the troops destined for Indochina would form the 25th Army of General Shōjirō Iida, with its headquarters in Saigon. These were the words of officers, not a government decision, but they showed how the army thought about the purpose of the whole operation. On 15 July the Army Chief of Staff, General George Marshall, presented the president with a memorandum on the intercepted reports of the imminent occupation of Indochina.

Magic was not the only source. On 5 July the British ambassador in Tokyo, Robert Craigie, expressed concern at the Japanese Foreign Ministry about the plans for a southward move, and the Japanese, surprised by the leak, briefly suspended their preparations. That same day the New York Times wrote of Japanese intentions to obtain bases in Indochina and Thailand, and State Department officials read the article to Nomura and asked how peace talks could be reconciled with preparations for war. In Tokyo the Soviet agent Richard Sorge passed on to Moscow what he had managed to learn of the decisions of the Japanese leadership.

An enquiry to Konoe

Hull’s telegram of 4 July was an attempt to find out which of the conflicting signals was true. Grew delivered it on 6 July to the prime minister’s secretary, Tomohiko Ushiba. In it the American government wrote that the reports of a Japanese intention to make war on the USSR were “so completely at variance” with the assurances of Nomura and other Japanese statesmen that it “found it difficult to believe” them, and that if Japan were to embark on a course of aggression and conquest, peace in the Pacific would prove an illusion. On 5 July the State Department officials Maxwell Hamilton and Joseph Ballantine informed Nomura of the telegram; in those days Nomura himself was warning Matsuoka, cabling on 3 July that if Japan now moved its army south, there would no longer be any hope of an understanding with the United States.

The reply came on 8 July, not from Konoe but from Matsuoka, who handed it to Grew on the prime minister’s behalf. Japan, he wrote, “has not so far considered the possibility of joining the hostilities” against the Soviet Union, and its attitude to that war was explained by its statement of 2 July to the Soviet ambassador, Konstantin Smetanin, a copy of which he enclosed. He in turn asked whether the president intended to enter the war in Europe. There was not a word in the reply about the south. It was formally true and at the same time evasive, and the Americans, reading Tokyo’s cables, knew that it passed over the most important thing in silence.

Matsuoka did not last long. His impetuousness, his talks with the Germans behind the prime minister’s back and his sabotage of the negotiations with Washington – on 14 July he rejected the latest American statement – exhausted Konoe’s patience. On 16 July the whole government resigned, and on 18 July Konoe’s third cabinet was formed, with Admiral Teijirō Toyoda as foreign minister. A cable from Tokyo to Berlin of 19 July, decrypted in Washington two days later, gave an assurance that the change of cabinet had “no further significance”, that foreign policy would not change and that Japan would remain faithful to the Tripartite Pact. The new government also confirmed to the French that its demands concerning Indochina remained in force.

Occupation, not yet attack

What Magic revealed in July 1941, then, was not a plan for an immediate attack on South-East Asia, but a plan to occupy southern Indochina, without fighting if possible, with the consent of a defenceless Vichy, and to build bases there from which it would one day be possible to strike further. Japan’s leaders counted on the United States responding with nothing more serious than a protest. Washington, knowing the cables, was not persuaded by assurances of peaceful intent. On 21 July the Acting Secretary of State, Sumner Welles, told the minister-counsellor of the Japanese embassy, Kaname Wakasugi, that according to information from many sources Japan would occupy southern Indochina by force in the next few days, which made the talks pointless. On 23 July, when Nomura explained that Japan feared being cut off from supplies of rice and raw materials and being “encircled”, Welles replied that the United States must regard this step as the last before expansion and conquest in the South Seas, and that Hull saw no basis for further talks. The next day Roosevelt proposed to Nomura the neutralisation of Indochina under a guarantee of the powers. The proposal did not stop the Japanese transports.

The limits of Magic

Magic gave the Americans exceptional knowledge of Japanese diplomacy, but not of Japanese military plans. Purple was the Foreign Ministry’s cipher. The army and navy had their own systems, and the fleet’s main operational code, called JN-25 by the Americans, could be read only to a small extent in 1941. The Foreign Ministry itself did not know the details of the military plans and did not share everything it knew with its embassies. The cables reached their readers late, piecemeal and without context, because no notes could be made from them. This made it possible to foresee the direction of Japanese policy, but not the day, place and manner of the blow. This limitation proved tragic in December 1941: the Americans knew that war was close, but they did not know that the first blow would fall on Hawaii.

Place in the war

The knowledge gained from the July cables shaped the American response. When at the end of July Vichy accepted the Japanese demands, Roosevelt froze Japanese assets in the United States on 26 July, and Britain and the Netherlands followed suit. On 28 July Japanese troops began landing in southern Indochina, and from 1 August American oil exports to Japan in practice ceased. Tokyo, which had not expected so sharp a reaction, faced a choice between concessions and war for the oil of the Dutch East Indies. On 6 September another Imperial Conference resolved to prepare for war if negotiations had produced no result by October. In October General Hideki Tōjō became prime minister, and on 7 and 8 December 1941 Japan struck at Pearl Harbor, Malaya, the Philippines and Hong Kong. The bases occupied in southern Indochina in July served as springboards for the attack on Malaya, and on 10 December aircraft from the airfields near Saigon sank the British warships HMS Prince of Wales and Repulse.

Throughout the war the Americans and the British read Japanese diplomatic correspondence. Particularly valuable were Ambassador Ōshima’s detailed reports from Berlin on his conversations with Hitler and on German fortifications, including those on the French coast before the Normandy landings. The Japanese learned that the Purple cipher had been broken only from the hearings of the congressional committee that investigated the attack on Pearl Harbor in 1945–1946. Most of what is known about the July 1941 cables comes from its records.