Franco - Thai War The Franco - Thai War ( ) ( 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 , 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 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 behaviour convinced the Phibun regime that Vichy France would not seriously resist a confrontation with Thailand . Opposing forces . French . French forces in Indochina consisted of an army of approximately 50 , 000 men , of whom 12 , 000 were French , organised into forty - one infantry battalions , two artillery regiments , and a battalion of engineers . The most obvious deficiency of the French army lay in its shortage of armour : it could only field 20 antiquated Renault FT tanks against nearly one hundred armoured vehicles of the Royal Thai Army . The bulk of the French forces stationed near the frontier with Thailand consisted of Indochinese troops of the 3rd and 4th " Tirailleurs Tonkinois " ( Tonkinese Rifles ) , together with a battalion of Montagnards , French regulars of the Colonial Infantry , and Foreign Legion units . The Armée de l ' Air had in its inventory approximately 100 aircraft , of which about 60 could be considered front line . These consisted of thirty Potez 25 TOEs , four Farman 221 s , six Potez 542 s , nine Morane - Saulnier M . S . 406 , and eight Loire 130 flying boats . Thai . The Thai Army was a relatively well - equipped force . Consisting of 60 , 000 men , it was made up of four armies . The largest was the Burapha Army , with five divisions . Independent formations under the direct control of the army high command included two motorised cavalry battalions , one artillery battalion , one signals battalion , one engineer battalion , and one armoured regiment . The artillery was a mixture of aged Krupp and modern Bofors howitzers and field guns , while 60 Carden Loyd tankette s and 30 Vickers six - ton light tanks made up the bulk of the army ' s tank force . The Royal Thai Navy was made up of two coastal defence ships , 12 torpedo boats , and four submarines . It was inferior to the French naval forces , 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 air force ' s first - line strength were 24 Mitsubishi Ki - 30 light bombers , nine Mitsubishi Ki - 21 medium bombers , 25 Hawk 75Ns pursuit planes , six Martin B - 10 medium bombers , and 70 O2U Corsair light bombers . Campaign . While nationalistic demonstrations and anti - French rallies were held in Bangkok , border skirmishes erupted along the Mekong frontier . The superior Royal Thai Air Force conducted daytime bombing runs over Vientiane , Phnom Penh , Sisophon , and Battambang with impunity . The French retaliated with their own planes , but the damage caused was less than equal . The activities of the Thai air force , particularly in the field of dive - bombing , was such that Admiral Jean Decoux , the governor of French Indochina , grudgingly remarked that the Thai planes seemed to have been flown by men with plenty of war experience . In early January 1941 , the Thai Burapha and Isan Armies launched their offensive on Laos and Cambodia . French resistance was instantaneous , but many units were simply swept along by the well - equipped Thai forces . The Thais swiftly took Laos , but Cambodia was more difficult to take . At dawn on January 16 , 1941 the French launched a large counterattack on the Thai - held villages of Yang Dang Khum and Phum Preav , initiating the fiercest battle of the war . Because of over - complicated orders and nonexistent intelligence , the French counterattacks were stopped and fighting ended with a French withdrawal from the area . The Thais were unable to pursue the retreating French , as their forward tanks were kept in check by the gunnery of French Foreign Legion artillery . With the situation on land deteriorating for the French , Admiral Decoux ordered all available French naval forces into action in the Gulf of Thailand . In the early morning of January 17 , the superior French fleet caught a Thai naval detachment at anchor off the island of Ko Chang . The subsequent battle of Ko Chang proved a victory for the French and resulted in the sinking of two Thai torpedo boats and a coastal defence ship . On January 24 , the final air battle took place when Thai bombers raided the French airfield at Angkor near Siem Reap . The last Thai mission to bombing at Phnom Penh commenced at 07 : 10 hours on January 28 , when the Martins of the 50th Bomber Squadron set out on a raid on Sisophon , escorted by thirteen Hawk 75Ns of the 60th Fighter Squadron . Results . Armistice . 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 General Philippe Pétain ' s Vichy France and the Kingdom of Siam were signed aboard the cruiser " Natori " on January 31 , 1941 , and a general armistice had been arranged to go into effect at 10 : 00 hours on January 28 . On May 9 a peace treaty was signed in Tokyo ( Tokyu Convention ) , with the French being coerced by the Japanese to relinquish their hold on the disputed border territories . France ceded the following provinces to Thailand from Cambodia : and from Laos : Treaty . The resolution of the conflict was received with wide acclaim among the Thai people and was looked upon as a personal triumph for Phibun . For the first time , Thailand had been able to extract concessions from a European power , albeit a weakened one . For the French in Indochina , the conflict was a bitter reminder of their isolation following the Fall of France . In the French view , an ambitious neighbour had taken advantage of a distant colony cut off from her weakened parent . Without hope of reinforcements , the French had little chance of offering a sustained resistance . However , the real beneficiaries of the conflict were the Japanese . They were able to expand their influence in both Thailand and Indochina . The Japanese won from Phibun a secret verbal promise to support them in an attack on British Malaya and British Burma . On 8 December 1941 , the Japanese invaded Thailand at the same time they invaded Malaya ( immediately after the attack on Pearl Harbor , Hawaii , USA ) . To commemorate the victory Phibun had the Victory Monument built . Thailand invited Japan and Germany to join the celebration . Japan ordered Shōjirō Iida to join the celebration and The German Foreign Ministry ordered Robert Eyssen to join the celebration . After the war , in October 1946 , northwestern Cambodia and the two Lao enclaves on the Thai side of the Mekong River were returned to French sovereignty after the French provisional government threatened to veto Thailand ' s membership in the United Nations . Casualties . The French army suffered a total of 321 casualties , of whom 15 were officers . The total number of men missing after January 28 was 178 ( 6 officers , 14 non - commissioned officers , and 158 enlisted men ) . The Thais had captured 222 men ( 17 North Africans , 80 Frenchmen , and 125 Indochinese ) . The Thai army suffered a total of 54 men killed in action and 307 wounded . 41 sailors and marines of the Thai navy were killed , and 67 wounded . At the Battle of Ko Chang , 36 men were killed , of whom 20 belonged to HTMS Thonburi , 14 to HTMS Songkhla , and 2 to HTMS Chonburi . The Thai air force lost 13 men . The number of Thai military personnel captured by the French amounted to just 21 . About 30 % of the French aircraft were rendered unserviceable by the end of the war , some as a result of minor damage sustained in air raids that remained unrepaired . The Armée de l ' Air admitted the loss of one Farman F221 and two Morane M . S . 406s destroyed on the ground , but in reality its losses were greater . In the course of its first experience of combat , the Royal Thai Air Force claimed to have shot down five French aircraft and destroyed 17 on the ground , for the loss of three of its own in the air and another five to 10 destroyed in French air raids on Thai airfields .