The Franco-Thai War was a brief interstate war that erupted in early 1941 when the nationalist military government of Thailand annexed parts of the French Indochinese colonial protectorate of Cambodia and then resumed again after the Second World War in early 1946 when the French forced their return.
The historical origins of the war lie in several centuries of Thai expansion at the expense of Cambodia and 19th French imperialism in Southeast Asia. France had granted the two provinces (Siem Reap province was then called Angkor province) to Thailand (Siam) in 1867 in return for the Kingdom of Thailand's renunciation of its partial sovereignty over the Kingdom of Cambodia, which then became a French Protectorate. The two provinces were restored to Cambodia in 1907.
The Fall of France in the June 1940 and the Japanese military occupation of Vietnam emboldened Thai nationalists/irredentists who wanted to annex Cambodia and Laos. Thailand had been ruled by the miltiary since the 1932 coup d'etat that overthrew the absolute monarchy. The conflict began with the distribution by the Thai miltiary government of leaflets calling for the "return" of both Cambodia and Laos. Neither Cambodia or Laos had ever been under unambigious and complete Thai political control and the cultural connections between Thailand and Cambodia hardly made them one nation. Although most Thais, Laotians and Cambodias share a belief in Theravada Buddhism and much of the Thai and Laotian high culture began in the Angkorian Empire in Cambodia, Khmer (the Cambodian language) is not related to any of the Thai or Laotian dialects. Khmer is instead an Australasian language related to Vietnamese and Mon. Aggressive nationalism rarely needs to take account of the facts however. Among the leading Thai nationalists were Nazi German and Japanese imperialist sympthasizers and "partisans of dictatorship" in their own country.
The war included aerial bombing and strafing raids on cities and villages by both the Thai and French colonial air forces. The Thais bombed Stung Treng, Battambang, Siem Reap, Sisophon, Ream and Mongkolborey. The French bombed Aranya and Ouborn. Offers of money were used by the Thais to encourage Cambodian and Laotian colonial troops to desert with their weapons and defect. Much of the fighting took the form of bloody skirmishes along the Thai-Cambodian border.
The most decisive battle in the war took place took place at sea. On January 17, 1941 the French Navy smashed the Thai Navy off Koh Chang Island and either sank or drove aground the Thai ships Dhonburi, Sri Ayuthia, Songkhla, Chonburi, and several Italian built PT boats.
In Japanese mediated negotiations in Tokyo in 1942, French Indochinese colonial officials were forced to cede all of Battambang province and parts of the provinces of Siem Reap, Stung Treng and Pursat. This represented a major portion of the rice production of French Indochina, the Pailin gem mines and part of the Tonle Sap Lake fisheries. The lost territories were renamed Phibunsongkhram province. The Japanese were hardly neutral parties in the negotiations. The Imperial Japanese Army wanted to use Thai territory for its military operations. The formal agreement was signed on March 11, 1941.
The British, who had helped restore French colonial rule in Indochina below the Sixteenth Parallel (southern Vietnam and the rest of Cambodia) at the end of hostilities against Japan, signed a declaration refusing to recognize the Thai annexations on January 1, 1945. Retrocession of the lost territories took place in 1946 following sporadic firefights between French colonial and Thai forces that may have included at least one naval bombardment of the Thai position at Svay Donkeo by a French gunboat on Tonle Sap Lake. The new border restoring the lost territories was signed in Paris on November 17, 1946.
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