In early June 1940, Thailand concluded a nonaggression pact with France. However, after Germany defeated France, the Thais lost interest in ratifying the pact. The pro-Japanese military government of Marshal Pibul Songgram saw an opportunity to capitalize on French weakness, and in October it called up military reservists.
The Thai army numbered 26,500 men, but reservists doubled it to nearly 50,000 men. The Thai air force had about 270 aircraft, of which 150 were combat types mostly of U.S. manufacture. The navy also had several dozen obsolete land-based aircraft, but in December 1940 Japan delivered 93 modern aircraft. The Thai navy had 10,000 men who manned royal yachts, a British-built World War I destroyer, and 2 British-built small gunboats and 8 motor torpedo boats (MTBs). Italy supplied 9 small torpedo boats, 2 minesweepers, and 9 minelayers. Two light cruisers under construction in Italy for Thailand were not available for the war and were indeed sequestered by Italy in 1941. Japan also delivered 2 armored coastal defense antiaircraft vessels, 4 small submarines, 2 escort/training ships, and 3 small torpedo boats. The Thai navy suffered from serious shortcomings. Its sailors were poorly trained, the older vessels were of limited value, the modern Italian torpedo boats were too flimsy for service in rough seas, and the Japanese submarines could not dive.
The French army in Indochina was comparable in size to the mobilized Thai force: 50,000 men. But 38,000 of these were Indochinese troops of questionable loyalty. The heart of the French military was the 5,000-man 5th Foreign Legion Regiment. The French possessed 30 World War I–vintage tanks. Much of their artillery was also outdated, and they were short of artillery ammunition. The French had fewer than 100 aircraft. French naval units in Indochinese waters were old and for the most part poorly armed. They consisted of the light cruiser La Motte-Picquet, 2 gunboats, 2 sloops, 2 auxiliary patrol craft, and several noncombatants.
France was extremely vulnerable. It had been defeated by Germany in Europe, and Japan was applying pressure in Indochina. In September 1940 Japanese troops invaded Tonkin, killed 800 French troops, and secured concessions and airfields. Bangkok decided to take advantage of these circumstances to reassert its claims.
In mid-November 1940, the Thais sent military units across the Mekong River into Cambodia, producing immediate military skirmishes with French troops. The French military was temporarily sidetracked by the 22 November Indochina Communist Party uprising in Cochin China, which it crushed in the first week of December. French High Commissioner of Indochina Admiral Jean Decoux then decided to go on the offensive. On land, a mixed French brigade attacked Thai positions at Yang Dom Koum on 16 January 1941. The assault failed because of insufficient manpower and insufficient heavy weapons. The Thais, who had planned their own offensive for the same day, then counterattacked. French Foreign Legionnaires beat back this attack, which included tanks. Both sides then withdrew from the immediate area, although Bangkok claimed victory.
At the same time, the French took the offensive at sea. The French navy planned an assault on the Thai naval detachment at Koh Chang and the principal Thai navy base at Sattahib. The initial strike was to be carried out by virtually the entire French flotilla: the cruiser, two gunboats, and two sloops. On January 16, the fleet sailed for the Gulf of Siam to Koh Chang, which guarded the passage to Sattahib. Early on 17 January, the French surprised the Thais. In the ensuing 90-minute action at Koh Chang, the French sank two Thai torpedo boats; they also sank a Thai coastal defense vessel and mortally damaged another. With no direct hits or losses to their own ships, the French then returned to Saigon on 19 January.
There was little air action during the war, although the Thais did use their Curtiss Hawk III biplanes in a dive-bombing role. The French also had a plan, which they did not implement, to fire-bomb Bangkok from the air.
The indecisive land and naval actions of January 1941 did not end the war. The Japanese then applied diplomatic pressure on France, threatening to intervene on the side of Thailand. On 31 January 1941 at Saigon, French and Thai officials signed a Japanese-dictated armistice aboard the Japanese cruiser Natori. In March the Vichy government, which was also being intimidated by Germany, accepted Japanese mediation. Under Japanese pressure, on 9 May 1941 France signed a peace treaty with Thailand in Tokyo whereby France transferred to Thai control three Cambodian and two Laotian provinces on the right bank of the Mekong River—about 42,000 square miles in area.
After the end of the World War II, France forced the Thais to return this territory in September 1945 and to accept the Mekong as the boundary separating Thailand from Laos and Cambodia to the east. But border clashes along the Mekong occurred in 1946 and with Cambodia as late as the 1980s.
Spencer C. Tucker
Decoux, Jean. A la Barre de l'Indochine. Paris: Plon, 1949.; Meisler, Jurg. "Koh Chang: The Unknown Battle: Franco-Thai War of 1940–41." World War II Investigator 2, no. 14 (1989): 26–34.; Mordal, Jacques, and Gabriel A. J. P. Auphan. La Marine Française pendant la Deuxième Guerre Mondiale. Paris: Hachette, 1958.; Mordal, Jacques. Marine Indochine. Paris: Amiot-Dumont, 1953.