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Unless otherwise indicated, all material researched, written, and copyrighted by Bill Stone for publication in various venues online and elsewhere. Feel free to point links at these pages, but reproduction of this material, electronic or otherwise, is prohibited without prior permission in writing from Stone & Stone.

Siam Goes to War

The Players

Neutral at the outbreak of the Second World War and traditionally friendly to the United Kingdom and United States, Siam by the time of the fall of France was becoming gradually more concerned about maintaining friendly relations with Japan. Aside from retaining its independence in the face of Japanese expansionism, the Siamese government grew increasingly obsessed with recovering the "lost provinces" ceded to Indo-China since the 19th century, and its diplomatic efforts focused on generating the necessary international support.

Japan viewed Siam and Indo-China as natural components of its "Greater East Asia Co-Prosperity Sphere". The fall of France presented immediate opportunities in Southeast Asia, but Japanese planning always recognized that its ambitions in that region must somehow reconcile the conflicting interests of Siam and Indo-China while simultaneously providing the maximum benefit to its long-term strategy against Britain. Not as overwhelmingly strong as its potential adversaries might believe, Japan proved surprisingly cautious in playing its hand.

Whatever the Japanese strength in the region, Western weakness was apparent to all. Given this military unreadiness, it was impossible to offer Siam -- and equally impossible for Siam to accept -- any Western military guarantees. This left only diplomatic and economic incentives in the Western arsenal. American policy took a rigid, righteous approach to maintaining the status quo in Southeast Asia, particularly in Indo-China which was recognized as very vulnerable to both Siam and Japan. Britain, on the other hand, was less concerned with idealism; the Foreign Office remained remorselessly intent on keeping a friendly, neutral Siam firmly interposed between Singapore and the Japanese armed forces, no matter what the cost. These policies diverged to the extent that, by the time of the border war between Siam and Indo-China, the two Western powers proved completely incapable of working together.

Indo-China held the worst cards of all: a restless native population tiring of its colonial masters; diplomatic and military pressure from Japan for major concessions; unrelenting Siamese irredentism; a weak military with little hope of reinforcement; complete lack of tangible support from metropolitan France; and strict expectations -- but without material sustenance -- from Britain and the US.

Siam Shuffles the Deck

Although Siam preferred to be bordered by a weak Vichy-controlled Indo-China rather than a strong Japanese-controlled colony, it seemed possible to Prime Minister Phibul to retain the former while nevertheless regaining control of the disputed border regions. In August 1940 Phibul explained to British Ambassabdor Sir Josiah Crosby that Siam felt unable to refuse a Japanese request for a military mission to travel to Tokyo for consultations. (In fact, as British signals intelligence revealed at the end of the month, the mission was actually at Phibul's request.) With that public gesture of Siamese-Japanese military goodwill, Phibul promptly and inaccurately informed the Vichy French that Japan intended to present Siam with the lost provinces in exchange for Siamese favors.

The US State Department, even though not informed of the intercepted signals, bluntly condemned the Siamese position, beginning a long spiral of worsening relations. Britain, on the other hand, decided that it was preferable to accept -- if not tacitly support -- Siamese claims against Indo-China rather than allow Siam to receive Japanese military assistance in the matter. The British position was taken on the assumption that a friendly Siam controlling only part of Indo-China could still provide the buffer that an unfriendly Siam and Japanese-controlled Indo-China could not.

On 28 September 1940, at the conclusion of the Vichy-Japanese clash at Lang Son and Haiphong, Phibul made further overtures to Tokyo, offering "full cooperation" with Japan. Japanese policy-makers then attempted to balance their new agreement with Vichy (which guaranteed Vichy sovereignty over all of Indo-China) with Siamese designs on Indo-Chinese territory and, enticingly, the prospect of advanced air and naval bases in Siam. In reality, though, as Japan soon realized, the Siamese only offered transit rights to Malaya in exchange for neutrality and territorial guarantees. The Japanese did not leap at the offer.

In October the first incidents began to occur on the frontier between Siam and Indo-China.

Understandably alarmed at the weakness of its position in Indo-China, Vichy attempted to dispatch reinforcements from Djibouti on the Red Sea to Saigon. Still convinced of its policy of quiet support for Siam, and equally convinced that the reinforcements were destined to resist Siam, not Japan, the British government instructed the Royal Navy to prevent such naval movements.

The Border War

By the third week of November widespread fighting and bombing was underway on the border and the Siamese army was mobilizing. Ambassador Crosby still hoped that Phibul could gain the contested territory without resorting to Japanese assistance. In the Foreign Office, the prevailing view was expressed quite exquisitely: "Stand back and see the dog fight, but speak soothingly to your favorite dog." Neither Bangkok nor London was pleased by the American decision to withhold delivery of aircraft for which Siam had already paid. As a result, a Siamese purchasing mission departed for Tokyo (although it was not until early January that Japanese weapons and tanks were being loaded for shipment to Bangkok).

During December further British intercepts indicated that Japanese-Siamese cooperation remained extremely limited. This intelligence was not shared with the Americans who continued to believe that Phibul was already in Tokyo's pocket, mistaking (not for the first time or the last) reckless nationalism for subservience to an enemy power.

As the border war continued to flare up, Tokyo and Bangkok sought common ground. Their differences were not small. Phibul wanted Japanese assistance in gaining territory from Indo-China and assurances that Siam could remain neutral in the coming war against Britain. Japan needed the opposite. It wanted no disturbance of agreements in Indo-China, but desired Siamese cooperation in the eventual struggle with Britain.

At the turn of the year Britain tried unsuccessfully to make a common front with the US and broker a settlement. On 17 January 1941 Vichy decisively won an important naval engagement at Koh-Chang. Two days later a Japanese conference decided -- despite pressure from the military to completely occupy both Indo-China and Siam -- to impose an end to the conflict. Hanoi was informed that the war must end or Japan would assume control of Indo-China.


Governor-General Decoux managed to postpone the arbitration conference. In the meantime, Siam inflated the size of its claims. The central government of Vichy in France prematurely and publicly accepted the demands, even though Japan had no intention of enforcing such an award. In the final settlement Siam was granted only a fraction of the territory coveted in Laos and Cambodia, and was further obliged to pay an indemnity to Vichy.

Most importantly, Japan convincingly demonstrated who held the balance of power in Southeast Asia.

This was not a settlement designed to placate Siam or bring that country to the bosom of Japan. Instead, it preserved order in Southeast Asia and permitted Japan to pursue long range goals of increasing its presence in Indo-China while gradually gaining access to Siam and then British possessions to the south.

The Siamese government realized that not only had the Japanese policy of "Asia for the Asiatics" proved a hollow lie in Indo-China, but Tokyo had played a skillful game to suit its own ambitions. There was as yet no chance Siam would commit to a military alliance with Japan based on mutual interests, but more than ever Phibul feared such an alliance would be imposed by the force of events beyond his control.

The Real War

On 8 December 1941 (7 December in the United States) Japanese troops landed by sea in Siam from newly acquired bases in southern Indo-China; they were resisted by Siamese police and local garrisons. The British "Krohcol" (LtCol H. D. Moorhead: 3/16 Punjab and 5/14 Punjab) -- belatedly executing Operation Matador -- crossed into Siam from Malaya at 3 PM on the same day and was resisted by "Siamese armed constabulary" forces and regular troops for the next few days until the column withdrew.

In the course of the previous 18 months the relative strengths of the opponents had been made abundantly clear to Phibul and the Siamese government. Phibul ordered his armed forces to cease all resistance against the Japanese.

Siam bowed to the inevitable, salvaging what it could, and concluded an agreement by which it maintained its sovereignty at the cost of unrestricted Japanese movement and access to facilities. On 21 December, with the Prince of Wales and Repulse sunk and the Japanese army advancing on Singapore, a formal treaty was signed in Bangkok at the Temple of the Emerald Buddha. In exchange for territorial concessions in Burma and Malaya, Siamese troops accompanied Japanese forces into the Shan States, and on 25 January 1942 Siam declared war on the United States and Britain.

On 10 May 1942 the Siamese "Phayap" (Northern) Army (MajGen Seri Roengrit: 2nd, 3rd, and 4th Divisions, total of approx 35,000 men) "launched an offensive" into Burma (they would have moved earlier, but the Japanese did not need their assistance and did not want to complicate their own plans) and engaged the withdrawing Chinese 93rd Army, on 26 May capturing Kengtung near the Sino-Burmese border.

Note on Names

The name of the Siamese Prime Minster, here transliterated as "Phibul" is also sometimes rendered as "Pibul" or "Phibun".

In the summer of 1939 the name Siam was changed to Thailand by decree of Prime Minister Phibul. Even so, the nation remained commonly known as Siam until after the war. I've stuck with the old name just for flavor.

Similarly, Indo-China has over the years gradually become Indochina, but I've stayed with the more common usage of the period. I employ the rather artificial "Vichy Indo-China" as a reminder that France and the colonial administration underwent a few changes in the summer of 1940.


Aldrich, Richard J. THE KEY TO THE SOUTH: BRITAIN, THE UNITED STATES, AND THAILAND DURING THE APPROACH OF THE PACIFIC WAR, 1929-1942. Kuala Lumpur: Oxford University Press, 1993.

Allen, Louis. BURMA: THE LONGEST WAR, 1941-45. New York: St Martin's Press, 1984

Hammer, Ellen J. THE STRUGGLE FOR INDOCHINA, 1940-1955. Stanford: Stanford University Press, 1966.

Hesse d'Alzon, Claude. LA PRESENCE MILITAIRE FRANCAISE EN INDOCHINE (1940-1945). Chateau de Vincennes: Publications du Service Historique de l'Armee de Terre, 1985.

Kirby, S. Woodburn. THE WAR AGAINST JAPAN, volume 1: THE LOSS OF SINGAPORE. London: HMSO, 1957.

Langer, William L. and S. Everett Gleason. THE UNDECLARED WAR, 1940-1941. New York: Harper & Brothers, 1953.

Reynolds, E. Bruce. THAILAND AND JAPAN'S SOUTHERN ADVANCE, 1940-1945. New York: St Martin's Press, 1994.

Copyright © 1998 by Bill Stone
May not be reproduced in any form without written permission of Stone & Stone



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