Could Japan Read Allied Signal Traffic? Japanese Codebreaking and the Advance into French Indo-China, September 1940

Abstract

The extent of Japanese codebreaking activities before Pearl Harbor has been unclear and many believed that the Japanese Army and Navy destroyed all documents on codebreaking in August 1945. Recently, however, the Japanese Navy records from 1940 were found at the archives of the Defense Agency of Japan. These documents show that the Japanese Navy's codebreaking team was able to read the British, US, French and Chinese diplomatic traffic in 1940. This essay will focus on how the Japanese decision makers exploited the available signal intelligence when Japan advanced into French Indo-China in September 1940.

Notes

Michael Barnhart, ‘Japanese Intelligence before the Second World War: “Best Case” Analysis’, in Ernest May (ed.) Knowing One's Enemy (Princeton, NJ: Princeton University Press 1986); J.W.M. Chapman, ‘Japanese Intelligence, 1918–1945: A Suitable Case for Treatment’, in Christopher Andrew and Jeremy Noakes (eds.) Intelligence and International Relations 1900–1945 (Exeter: Exeter University Press 1987); Hisashi Takahashi, ‘A Case Study: Japanese Intelligence Estimates of China and the Chinese 1931–1945’, in The Intelligence Revolution: A Historical Perspective (US Air Force Academy October 1988); Edward Drea, ‘Reading Each Other's Mail: Japanese Communication Intelligence 1920–1941’, The Journal of Military History 55/2 (April 1991) pp.185–205.

Rutland, who had been hired by the Japanese Navy in 1933, gathered information on the US Navy staying in Los Angeles and Hawaii, but his activities had been under surveillance by MI5 (British Security Service). He was arrested by MI5 in October 1941, just before the Japanese attack on Pearl Harbor; KV 2/328–38, PRO (Public Record Office, Kew, UK).

See Keiichiro Komatsu, Origins of the Pacific War and the Importance of ‘Magic’ (Surrey: Japan Library 1999); Richard Aldrich, Intelligence and the War against Japan (Cambridge: Cambridge University Press 2000); Michael Smith, The Emperor's Codes (Kent: Bantam Press 2000); Roland Worth, Jr., Secret Allies in the Pacific (North Carolina: McFarland & Company 2001).

After the outbreak of the Pacific war, ONI (Office of Naval Intelligence) and MIS became responsible for Japanese Navy and Army intelligence respectively. MIS was good at making an analysis of Japanese codebreaking.

The Japanese Intelligence System, 4 September 1945, RG 457 Entry 9002 Box 90 SRH254, NARA (National Archives and Records Administration, Washington DC).

The CIA found that British diplomatic traffic from Craigie to Eden on 30 September 1941 was decoded by the Japanese; Fred C. Woodrough, Jr. to Vice Admiral Rufus L. Taylor, USN, Deputy Director of Central Intelligence, 28 November 1967, ‘Reports on Items Based on Material Contained in the Archives in the Japanese Ministry of Foreign Affairs, 1868–1965’, NSA files, NARA.

Chapman, ‘Japanese Intelligence’ (note 1) p.148.

Los Angeles Times, 7 December 2001; Toshihiro Minohara, ‘Nichi-Bei Ango Sensou to Seisakukettei heno Eikyo’ (The Japanese-American Codebreaking War and its Influence on the Japanese Decision Making), Gaiko Forum 174 (January 2003) pp.76–81.

These documents were dated from 18 May 1941 to 6 December 1941 and 16 US, 5 Chinese, 3 British and 2 Canadian ciphers. The documents are available on the website, http://www.jacar.go.jp/index.htm (Japan Center for Asian Historical Records, National Archives of Japan).

Tsutao Ariga, Nihon Riku Kai Gun No Jyohokiko To Sonokatudo (The Japanese Army and Navy Intelligence Organization and their Activities) (Tokyo: Kindai Bungei-sha 1994) p.150.

Ibid., pp.276–9.

Kenichi Nakamuta, Jo-ho Shikan no Kaisou (The Memoir of Intelligence Officer) (Tokyo: Diamond-sha 1974) p.84.

Ariga, Nihon Riku (note 10) pp.309–14, Gray was also read by the GC&CS; Richard Aldrich, Intelligence and the War against Japan (Cambridge: Cambridge University Press 2000) p.82.

 > From my personal point of view, ‘ID7’ could be assumed to be the British Interdepartmental code.

‘BJ’ stands for Blue Jacket, generally named decrypted foreign documents, while ‘Magic’ stood for only decrypted Japanese documents.

Sigaba had been used by the US military services from 1938 to 1960s and Typex had been used by the British military services and Foreign Office throughout the war. Axis countries failed to break them.

Takagi's diary dated from January 1939 to August 1944 retained copies of decrypted British and American messages; Takagi Sokichi Nikki (Diary of Sokichi Takagi) (Tokyo: Mainichi Sinbunsha 1985).

Chuichi Ohnashi, Taiheiyo Senso Yuraiki (The Origins of the Pacific War) (Tokyo: Yumani Syobo 2002) p.54.

Chihiro Hosoya, ‘Nihon no Ei-Bei Kan to Senkanki no Higashi Asia’ (Japanese Perspective of Britain and the US and the East Asia in the Inter-war Period), in Chihiro Hosoya (ed.) Nichi-Ei Kankeishi 1917–1949 (The History of Anglo-Japanese Relations) (Tokyo: Tokyo University Press 1982) p.29.

This point is discussed by Edward Dreyer, China at War 1902–1949 (London: Longman 1995), pp.239–45.

Nicholas Tarling, Britain, Southeast Asia and the Onset of the Pacific War (Cambridge: Cambridge University Press 1996).

Negotiations between Japan and French Indo-China were described as follows: Eric Jennings, Vichy in the Tropics (Stanford: Stanford UP 2000); John Dreifort, Myopic Grandeur (Kent: Kent State UP 1991); Martin Thomas, The French Empire at War (New York: Manchester UP 1998).

COS (39) 941, 11 July 1940, CAB 53/52, PRO.

COS (40) 592, 31 July 1940, CAB 66/10, PRO.

Toku-jo, Bei (US) 1719, 11th Section, Navy General Staff (7552), 3 July 1940, Futsuin.

Sanbo Honbu Hen, Sugiyama Memo, ed. Army General Staff (Tokyo: Hara Shobo 1969) pp.10–15.

Paul Baudouin, The Private Diaries of Paul Baudouin (London: Eyre and Spottiswoode 1948), p.199.

BJ 082514, 5 August 1940, HW 12/255, PRO.

WM (40) 221, 7 August 1940, CAB 66/10, PRO.

Nicholas Tarling, ‘The British and the First Japanese Move into Indo-China’, Journal of Southeast Asian Studies XXI/1 (1990), p.45.

BJ082724, 15 August 1940, HW 12/255, PRO.

The Times, 17 August 1940.

Tarling, ‘The British’ (note 30) p.51.

WM (241)40, CAB 65/9, 4 September 1940, PRO.

Toku-jo, Bei (US) 2488, 11th Section, Navy General Staff (13879), 7 September 1940, Futsuin.

Toku-jo, Ei (UK) 1874, 11th Section, Navy General Staff (8984), 9 September 1940, Futsuin; see also Henderson to Halifax, 6 September 1940, F4163/3429/61, FO 371/24719, PRO.

Toku-jo, Ei 1884, 11th Section, Navy General Staff (9009), 9 September 1940, Futsuin; see also Telegram, 8 September 1940, F4204/3429/61, FO 371/24719, PRO.

Boeicho Boeikenkyu-jo (eds) (The National Institute for Defense Studies, Japan Defense Agency), Daitoa SenSo Kaisen Keii (The Origins of the Great East Asian War), Vol. 2, (Tokyo: Asagumo, 1973), p.68.

Boeicho, p.70.

Toku-jo, Si (China) 8640, 11th Section, Navy General Staff (6284), 9 September 1940, Futsuin; see also Minute by Bulter, 5 September 1940, F4219/3429/61, FO 371/24719, PRO.

Toku-jo, Ei 1882, 11th Section, Navy General Staff (9006), 10 September 1940, Futsuin; see also Halifax to Kerr, 7 September 1940, F4163/3429/61, FO 371/24719, PRO.

Nishihara to Tokyo, 11 September 1940, Futsuin Sinchu Mondai (French Indo-China Proceeding Problems), Vol.2, Asian History Records Center.

Matsuoka to Suzuki, 11 September 1940, Futsuin Sinchu Mondai (French Indo-China Proceeding Problems), Vol.2, Asian History Records Center.

Richard Aldrich, The Key to the South (Oxford: Oxford UP 1993) p.266.

Toku-jo, Ei 1881, 11th Section, Navy General Staff (8985), 10 September 1940, Futsuin; see also Telegram from Crosby, 9 September 1940, F4158/19140, FO 371/24751, PRO.

Toku-jo, Bei 2585, 11th Section, Navy General Staff (14211), 12 September 1940, Futsuin.

Jun Tsunoda (ed.) Gendaishi Shiryo (Documents on Contemporary History) (10) Nitchu Senso III (The Sino-Japanese War) (Tokyo: Misuzu Shobo 1964) p.388.

Toku-jo, Ei 1881.

BJ 083518, 17 September 1940, HW 12/256, PRO.

Futsu-In.

Toku-jo, Ei 1428, 11th Section, Navy General Staff (14211), 17 September 1940, Futsuin Mondai; see also Halifax to Lothian, 14 September 1940, F4229/3429/61, 15 September 1940, F4163/3429/61, FO 371/24719, PRO.

Toku-jo, Si 2775, 11th Section, Navy General Staff (14211), 18 September 1940, Ei 1406, 11th Section, Navy General Staff (9188), Futsuin Mondai; see also, Telegram from Clark Kerr, 13 September 1940, F4204/3429/61, FO 371/24719, PRO.

Matsuoka to Suzuki, 19 September 1940, Futsuin Sinchu Mondai, Vol.2, Asian History Records Center.

BJ 083584, 20 September 1940, HW 12/256, PRO.

Eden to Amery, 21 September 1940, F 4248/G, FO 371/24719, PRO.

Tokyo to Foreign Office, 16 September 1941, F 4126/3429/61, FO 371/24719, PRO.

Boeicho, p.93.

Cordell Hull, The Memoirs of Cordell Hull, vol.1 (New York: Macmillan 1948) p.906.

Toku-jo, Ei 1452, 11th Section, Navy General Staff (9887), 21 September 1940, Futsuin Mondai, See also; Telegram 16 September 1940, F4204/3429/61, FO 371/24719, PRO.

Hull, Cordell Hull (note 58) p.905, ‘Sinajihen Futsuryo Indo China Sinchumondai’ (Problems of the Move into French Indo-China).

Toku-jo, Bei 2657, 11th Section, Navy General Staff (15780), 20 September 1940, Futsuin.

Hull, Cordell Hull (note 58) p.907.

Toku-jo, Ei 1489, 11th Section, Navy General Staff (9455), 26 September 1940, Futsuin.

Toku-jo, Bei 2673 and 2685, 11th Section, Navy General Staff (15813),(16125), 22 September 1940.

Smith, The Emperor's Codes (note 3) pp.79–89. According to the records of PRO, the GC&CS broke ‘Purple’ on 15 February; BJ 087742, 087743, 15 February 1941, HW12/261, PRO.

BJ 092889, 4 July 1941, HW12/266, PRO.

Telegram, 4 July 1941, F5883/523/G, FO 371/27892, Eden to Craigie, 4 July 1941, FO 371/27892, PRO. The Japanese government was proud of the preservation of the secret plan to advance into Indo-China in July 1941, but Craigie warned the Japanese Foreign Office about Japanese intentions on Indo-China based on BJ information. The General Staff were shocked by Craigie's visit and Matsuoka decided to postpone a negotiation with Indo-China. Vice Foreign Minister Ohashi wrote in his memoir, ‘How could Britain know our secret plan of French-Indo China!’ Gunji Shigaku Kai Hen, Kimitsu Sensou Nisshi (Secret War Diary), ed. The Military History Society of Japan (Tokyo: Kinsensha 1998) p.127; Ohashi, p.165.

See Komatsu, Origins of the Pacific War (note 3); Department of Defense, United States of America, The “Magic” Background of Pearl Harbor, vols.1–5 (Washington: US Government Printing Office 1977); Herbert Feis, The Road to Pearl Harbor (Princeton: Princeton UP 1950).

BJ 089068, 25 March 1941, HW 12 /262, PRO.

The Gaiko Shiryokan possesses decrypted British telegrams dated 18 and 24 May and 26 August. It could be assumed that these telegrams were sent by the FEK cipher, which the Japanese codebreaking team was able to break; Toku-jo (Special Information), Nichi-Bei Gaikou Zassan (Diplomatic Records of Japanese–American Relations), Gaimusyo Gaiko Shiryokan; see also, Halifax to Eden, 17 May 1941, F4178, FO 371/27880, Craigie to Eden, 24 May 1941, F4457/12/23, FO 371/27880, Craigie to Eden, 26 August 1940, F8410, FO 371/27892, PRO.

BJ 095346, 11 September 1941, HW 12/268, PRO.