The Franco-Thai War (October 1940 – 28 January 1941, Thai: กรณีพิพาทอินโดจีน, romanized: Karani Phiphat Indochin; French: Guerre franco-thaïlandaise) 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.

The French military forces in Indochina consisted of an army 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.

The Vichy French Navy had the light cruiser Lamotte-Picquet and four 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.

The slightly larger Thai Army was a relatively well-equipped force. Consisting of 60,000 men, it was made up of four armies. The largest were the "Burapha Army" with five divisions and the "Isan Army" with three divisions. 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, 12 torpedo boats, and four Japanese-made submarines. The Royal Thai Navy was inferior to the French naval forces in Southeast Asia, but 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.