The Franco-Thai War (October, 1940– 9 May, 1941) was fought between Thailand (Siam) and France over certain areas of French Indochina.
Negotiations with France shortly before World War II had shown that the French government was willing to make appropriate changes in the boundaries between Thailand and French Indochina, but only slightly. Following the Fall of France in 1940, Major-General Plaek Pibulsonggram (popularly known as “Phibun”), the prime minister of Thailand, decided that France’s defeat gave the Thais an even better chance to regain the territories they had lost during King Chulalongkorn’s reign.
The German military occupation of a large part of France made France’s hold on its overseas possessions, including Indochina, difficult. The colonial administration was now cut off from outside help and outside supplies. After the Japanese invasion of Indochina in September 1940, the French were forced to allow Japan to set up military bases. This seemingly subservient behaviour convinced the Phibun regime that France would not seriously resist a confrontation with Thailand.
Japan stepped in to mediate the conflict. A Japanese-sponsored “Conference for the Cessation of Hostilities” was held at Saigon and preliminary documents for a cease-fire between the governments of Marshal Philippe Pétain’s France and the Kingdom of Siam were signed aboard the cruiser Natori on 31 January, 1941, and a general armistice had been arranged to go into effect at 10:00 hours on 28 January. 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.