The Franco-Thai War (Thai: กรณีพิพาทอินโดจีน French: Guerre franco-thaïlandaise) (1940–1941) was fought between Thailand and Vichy France over certain areas of French Indochina that had once belonged to Thailand.
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. 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 occupation of metropolitan France made France's hold on its overseas possessions, including Indochina, tenuous. The isolated colonial administration was 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 behavior convinced the Phibun regime that Vichy France would not seriously resist a confrontation with Thailand.