The Franco-Thai War (October 1940 – 28 January 1941, ; ) was fought between Thailand and Vichy France over certain areas of French Indochina.
Negotiations shortly before World War II had shown that the French government was willing to alter the boundaries between Thailand and French Indochina, but only slightly. Following the Fall of France in 1940, Major-General Plaek Phibunsongkhram (popularly known as "Phibun"), the prime minister of Thailand, decided that France's defeat strengthened the Thais' negotiating position to regain the vassal state territories that were ceded to France during King Chulalongkorn's reign.
The German and Italian military occupation of Metropolitan France rendered France's hold on French Indochina and its other overseas territories tenuous. The colonial administration was cut off from outside help and supplies. After the invasion of French Indochina in September 1940, Japan forced the French to allow them to set up military bases. The seemingly subservient behavior of the French lulled the Phibun regime into believing that France would not seriously resist a military confrontation by Thailand.
Background The events of Franco-Siamese Crisis of 1893 (Rattanakosin Era R.S.112) and Franco-Siamese Treaty of 1907 ended with Siam losing the territories of Laos and Cambodia to France. The crisis was seen as a national humiliation according to "Lost Territories" narrative but available political actions and negotiation attempt between Siam and Western Powers were limited.
After Siamese Revolution and Boworadet Rebellion Siam were under Khana Ratsadon control. However, the uneasy alliance between Civilian and Military wings of Khana Ratsadon led to the consolidation of political and military powers to the militarist wing under Colonel Phraya Phahonphonphayuhasena in June 1933 Siamese coup d'état.
On 3 October 1936, Marcel Rey - French Minister in Siam have reported about Siamese newspaper publishing irredentist articles about the French occupation of Cambodian provinces while the Siamese government has voted for a credit of 30 million Baht for over five years for strategic road construction towards Indochinese border.Barthel, Alexandre.(2024) French Appeasement Policy Toward Siam (1937-1939). Asian Review Vol. 37(2), 2024. page 70
In Early 1939, French Indochina, seeing Siam as a threat, tries to appease Siam by exchanging military missions while noting the increase of Siamese military movements and Japanese officials in the northeastern regions (Isan) of Siam.Barthel, Alexandre.(2024) French Appeasement Policy Toward Siam (1937-1939). Asian Review Vol. 37(2), 2024. page 74
Later Phraya Phahon would make way for Minister of Defense Plaek Phibunsongkhram as a new leader of the militarist wing of Khana Ratsadon Government on 16 December 1938 making Phibun the new Prime Minister of Siam. Phibun then assigned Luang Wichitwathakan to implement state propaganda towards reclaiming lost territories from French Indochina and British Burma and reunite Tai Race under newly created "Thailand" (Renamed from Siam to Thailand on 23/24 June 1939).
During the Battle of France, the French government has decided to sign a non-aggression treaty with Thailand on 12 June 1940 regarding of mutual respect on Thai and French territorial integrity with a joint commission to inspect the borders between Thailand and French Indochina just few days before the total collapse of Metropolitan France to the invading German forces on 17 June 1940.Office of the Historian.(1945). The Acting Secretary of State to the Ambassador in France (Caffery). Foreign Relations of the United States, Diplomatic Papers, 1945, The British Commonwealth, The Far East, Volume VI At the same time Thailand also signed a non-aggression treaty with the British Empire and Japanese Empire on 12 June resulting in Anglo-Thai Non-Aggression Pact of 1940 and Treaty between Thailand and Japan Concerning the Continuance of Friendly Relations and the Mutual Respect of Each Other's Territorial Integrity respectively.
Fall of France and the Armistice of 22 June 1940 led to the creation of Vichy Regime in Southern France. Vichy government then inherited its control over French colonial territories in both Africa and Asia. With weakened status of the French state both Phibun and the Japanese Empire saw opportunities in Indochina. For Thais to reclaim their lost territories and for the Japanese to create a starting point for their conquest of Southeast Asia.
Late September 1940, Japanese forces launched attacks on French Indochinese positions along the border with China supported by aircraft from aircraft carriers in Gulf of Tonkin area. Lạng Sơn is captured by the Japanese troops thus starting the Japanese Invasion of Indochina. An invasion would gradually progress in phases from September to December 1940 but an armed military actions only contained in September 1940.
Phibun personally gave a speech at the student's political rally on 8 October 1940 in front of Ministry of Defence and the Grand Palace for Chulalongkorn and Thammasat students with cheering crowds who came to support the cause for reclaiming Indochinese territories from France followed by an oath taking ceremony.
Forces French The Vichy French Army in Indochina consisted of approximately 50,000 men, 12,000 of whom were French, organized into forty-one infantry battalions, two artillery regiments, and a battalion of engineers. The Vichy French Army had a shortage of armor, and it could field only 20 Renault FT tanks against the nearly one hundred Royal Thai Army armored vehicles. The bulk of the French forces stationed near the Thai border consisted of the Indochinese infantry of the 3rd and 4th Regiments of Tonkinese Rifles (Tirailleurs Tonkinois), together with a battalion of Montagnards (indigenous Vietnamese highlanders), French regulars of the Colonial Infantry (Troupes coloniales), and French Foreign Legion units.Rives, Maurice. Les Linh Tap. page 90
The Vichy French Navy only had the light cruiser Lamotte-Picquet and four small avisos in French Indochina.
The Vichy French Air Force (Armée de l'Air) had approximately 100 aircraft, of which roughly 60 could be considered front-line. These included thirty Potez 25 TOE reconnaissance/fighter-bombers, four Farman 221 heavy bombers, six Potez 542 bombers, nine Morane-Saulnier M.S.406 fighters, and eight Loire 130 reconnaissance/bomber flying boats.
Thai
The slightly larger Royal Thai Army was a relatively well-equipped force. Consisting of 60,000 men, it was made up of three armies. The largest were the "Burapha Army" with five divisions, the "Isan Army" with three divisions and "Independent formations" under direct control of the army high command included two motorized cavalry battalions, one artillery battalion, one signals battalion, one engineer battalion, and one armored regiment. The artillery was a mixture of Krupp guns and modern Bofors guns and howitzers, while 60 Carden Loyd tankettes and 30 Vickers 6-ton tanks made up the bulk of the army's tank force.
The Royal Thai Navy included two Thonburi-class coastal defense ships, HTMS Thonburi and HTMS Sri Ayudhya, 12 torpedo boats, and four Japanese-made submarines.Young, Edward M. (1995) Aerial Nationalism: A History of Aviation in Thailand. Smithsonian Institution Press. In spite of its numerical superiority and more recent boats, Royal Thai Navy was arguably inferior to the French naval forces in Southeast Asia.
The Royal Thai Air Force held both a quantitative and qualitative edge over the local Armée de l'Air units. Among the 140 aircraft that composed the Royal Thai Air Force's first-line strength were 24 Mitsubishi Ki-30 light bombers, nine Mitsubishi Ki-21 heavy bombers, 25 Curtiss Hawk 75N fighter planes, six Martin B-10 medium bombers, and 70 Vought O2U Corsair observation/attack aircraft.Royal Thai Air Force. (1976) The History of the Air Force in the Conflict with French Indochina. Bangkok.
Campaign
While nationalist demonstrations and anti-French rallies, supported by groups such as Khana Lueat Thai, were being held in Bangkok, several border skirmishes erupted along the Mekong frontier. The superior Royal Thai Air Force then conducted daytime bombing runs over military targets in Vientiane, Phnom Penh, Sisophon, and Battambang with impunity. The French retaliated with their own air attacks, but the damage they caused was less than equal. The activities of the Thai air force, particularly in the field of dive-bombing, was such that Admiral Jean Decoux, the governor of French Indochina, grudgingly remarked that the Thai planes seemed to have been flown by men with plenty of combat experience.Elphick, Peter. (1995) Singapore: the Pregnable Fortress: A Study in Deception, Discord and Desertion. Coronet Books.
On 5 January 1941, following the report of a French attack on the Thai border town of Aranyaprathet, the Thai Burapha and Isan Armies launched an offensive on Laos and Cambodia. French response was instantaneous, but many units were simply swept aside by the better-equipped Thai forces. The Thai army swiftly overran Laos, but the French forces in Cambodia managed to rally and offer more resistance.Vichy versus Asia: The Franco-Siamese War of 1941
At dawn on 16 January, the French launched a large counterattack on the Thai-held villages of Yang Dang Khum and Phum Preav, initiating the fiercest battle of the war. Due to poor coordination and nonexistent intelligence against the entrenched and well-prepared Thai forces, the French operation was stopped and fighting ended with a French retreat from the area. However, the Thais were unable to pursue the retreating French, as their forward tanks were kept in check by the gunnery of French Foreign Legion artillery.
With the situation on land rapidly deteriorating for the French, Admiral Decoux ordered all available French naval forces into action in the Gulf of Thailand. In the early morning of 17 January, a French naval squadron caught a Thai naval detachment by surprise at anchor off Ko Chang island. The subsequent Battle of Ko Chang was a tactical victory for the French and resulted in the sinking of two Thai torpedo boats and the disabling of a coastal defense ship, with the French suffering no casualties."The Battle of Koh Chang (January 1941)" netmarine.net
On 24 January, the final air battle took place when Thai bombers raided the French airfield at Angkor, near Siem Reap. The last Thai bombing mission on Phnom Penh commenced at 07:10 on 28 January, when the Martins of the 50th Bomber Squadron set out on a raid on Sisophon, escorted by thirteen Hawk 75Ns of the 60th Fighter Squadron. Japan wanted to maintain both its working relationship with Vichy and Thai, because Japanese intention was to use Thailand and Indochina as their military base to invade Burma and Malaya. The Japanese intervened, proposing an armistice between Thai and Vichy French be signed.
Armistice
Japan subsequently stepped in to mediate the conflict. A general ceasefire had been arranged to go into effect at 10:00 on 28 January, and a Japanese-sponsored "Conference for the Cessation of Hostilities" was held at Saigon, with preliminary documents for an armistice between the governments of Marshal Philippe Pétain's French State and the Kingdom of Thailand signed aboard the cruiser Natori on 31 January 1941. On 9 May, a peace treaty was signed in Tokyo, with the French being coerced by the Japanese to relinquish their hold on the disputed border territories. France ceded the following provinces to Thailand from Cambodia and Laos:
Battambang and Pailin, which were reorganized as Phra Tabong Province Siem Reap, Banteay Meanchey and Oddar Meanchey, which were reorganized as Phibunsongkhram Province Preah Vihear, which was merged with a part of Champassak Province of Laos opposite Pakse to form Nakhon Champassak Province