The 'Franco-Thai War' was fought between Thailand and Vichy France for possession of certain areas of French Indo-China (October 1940/28 January 1941).

Negotiations with France shortly before World War II had shown that the French government was willing to make changes, albeit of only a limited nature, in the boundary between Thailand and French Indo-China. Following the fall of France to Germany in June 1940, General Plaek Pibulsonggram, the Thai prime minister, came to the conclusion that France’s defeat had offered Thailand an even better chance to regain the vassal state territories that had been ceded to France in the late 19^th century during King Chulalongkorn’s reign.

The German military occupation of metropolitan France rendered France’s hold on its overseas possessions, including French Indo-China, tenuous. The colonial administration was isolated from outside help and sources of supply. After the Japanese 'Fu' invasion of French Indo-China in September 1940, the French had been forced to allow Japan to establish military bases, and this apparently subservient behaviour lulled the Pibulsinggram régime into believing that France would not, or indeed could not, seriously resist a military confrontation with Thailand.

The Vichy French military forces in Indo-China comprised an army of some 50,000 men, of whom 12,000 were French, organised into 41 infantry battalions, two artillery regiments, and one battalion of engineers. The Vichy French army had a shortage of armour, and it could field only 20 elderly Renault FT light tanks against the Royal Thai army’s total of nearly 100 armoured vehicles. The bulk of the French forces stationed near the Thai border consisted of the Indo-Chinese infantry of the 3^rd and 4^th Regiments of Tonkinese Rifles (Tirailleurs Tonkinois), together with one battalion of Montagnards (indigenous Vietnamese highlanders), French regulars of the Colonial Infantry (Troupes coloniales), and French Foreign Legion units.

The Vichy French navy had one light cruiser and four colonial sloops in French Indo-China.

The Vichy French air force had about 100 aircraft, of which roughly 60 could be considered front-line machines. These included 30 Potez 25TOE twin-engined reconnaissance/fighter-bombers, four Farman F.221 four-engined heavy bombers, six Potez 542 twin-engined bombers, nine Morane-Saulnier MS.406 single-engined fighters, and eight Loire 130 single-engined reconnaissance/bomber flying boats.

The slightly larger Royal Thai army was a relatively well-equipped force of some 60,000 men in 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 motorised cavalry battalions, one artillery battalion, one signals battalion, one engineer battalion and one armoured regiment. The artillery was a mixture of obsolescent Krupp guns and modern Bofors guns and howitzers, and 60 Carden Loyd tankettes and 30 Vickers 6-Ton Tanks constituted the bulk of the army’s tank force.

The Royal Thai navy included two 'Thonburi' class coast-defence ships, 12 torpedo boats, and four Japanese-made submarines. The Royal Thai navy was inferior to the French naval forces in South-East 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 constituted the Royal Thai air force’s first-line strength were 24 Mitsubishi Ki-30 'Ann' twin-engined light bombers, nine Mitsubishi Ki-21 'Sally' medium bombers, 25 Curtiss Hawk 75N single-engined fighters, six Martin B-10 twin-engined medium bombers, and 70 Vought O2U Corsair single-engined observation/attack aircraft.

While nationalist demonstrations and anti-French rallies were being held in Bangkok, several border skirmishes erupted along the Mekong river frontier. The Royal Thai air force then undertook the daytime bombing of military targets in Vientiane, Phnom Penh, Sisophon and Battambang with impunity. The French retaliated with their own air attacks, but the damage they caused was less than that inflicted by the Thais. The activities of the Royal Thai air force, particularly in the field of dive-bombing, was such that Vice-amiral d’escadre Jean Decoux, the governor of French Indo-China, grudgingly remarked that the Thai warplanes seemed to have been flown by men with plenty of war experience.

On 5 January 1941, following the report of a Vichy French attack on the Thai border town of Aranyaprathet, the Burapha Army and Isan Army launched an offensive into Laos and Cambodia. The Vichy French response was rapid, but many units were simply swept aside by the better-equipped Thai forces. The Thai army swiftly overran Laos, but the French forces in Cambodia managed to rally and offer more resistance.

At dawn on 16 January, the Vichy French launched a major counterattack on the Thai-held villages of Yang Dang Khum and Phum Preav, initiating the fiercest battle of the war. As a result of poor co-ordination and nonexistent intelligence against the entrenched and well-prepared Thai forces, the Vichy French operation was checked and fighting ended with a Vichy French retreat from the area. However, the Thais were unable to pursue the retreating Vichy French as their armour was kept in check by the gunnery of Foreign Legion’s artillery.

With the Vichy French situation on land rapidly deteriorating, Decoux ordered all available French naval forces into action in the Gulf of Thailand. In the early morning of 17 January, a Vichy French naval squadron caught a Thai naval detachment by surprise at anchor off Ko Chang island. The subsequent 'Battle of Ko Chang' was a tactical victory for the Vichy French and resulted in the sinking of two Thai torpedo boats and the disabling of a coast-defence ship, with the French suffering no casualties. Concerned that the war would now turn in Vichy France’s favour, the Japanese intervened with the proposal for an armistice.

On 24 January, the final air battle took place when Thai bombers raided the Vichy French airfield at Angkor, near Siem Reap. The last Thai mission bombing Phnom Penh began at 07.10 on 28 January, when the B-10 bombers of the 50^th Bomber Squadron set out on a raid on Sisophon, escorted by 13 Hawk 75N fighters of the 60^th Fighter Squadron.

Under Japanese auspices, a general ceasefire had been arranged to go into effect at 10.00 on 28 January, and a Japanese-sponsored 'Conference for the Cessation of Hostilities' was held in Saigon, with preliminary documents for an armistice between the governments of Maréchal de France Philippe Pétain’s Vichy French state and the kingdom of Thailand signed aboard the cruiser Natori on 31 January 1941. On 9 May, a peace treaty was signed in Tokyo after the Vichy French had been coerced by the Japanese to relinquish their hold on the disputed border territories. France ceded the following Cambodian and Laotian provinces: Battambang and Pailin, which were reorganised as Phra Tabong province; Siem Reap, Banteay Meanchey and Oddar Meanchey, which were reorganised as Phibunsongkhram province; Preah Vihear, which was merged with a part of Champassak province of Laos opposite Pakse to form Nakhon Champassak province; and Xaignabouli, including part of Luang Prabang province, which was renamed Lan Chang province.

The resolution of the conflict was widely acclaimed by the people of Thailand, and was seen as a personal triumph for Phibunsongkhram. For the first time in its history, Thailand had been able to extract concessions from a European power, albeit a weakened one. For the Vichy French in Indo-China, the conflict was a bitter reminder of their isolation after the fall of France. They felt that an ambitious neighbour had taken advantage of a distant colony isolated from a weakened parent. Without hope of reinforcement, the Vichy French had little chance of offering a sustained resistance.

The Japanese wished to maintain both their working relationship with Vichy France and with the status quo, so the Thais were forced to accept only a quarter of the territory that they gained from the French, in addition to having to pay 6 million piastres as a concession to the Vichy French.

However, the real beneficiaries of the conflict were the Japanese, who were able to expand their influence in both Thailand and Indo-China. The Japanese desired to use Thailand and Indo-China as bases for the planned invasions of Burma and Malaya. The Japanese won from Phibunsongkhram a secret verbal promise to support them in these attacks, but in the event Phibunsongkhram did not keep his word.

Relations between Japan and Thailand were subsequently stressed, as a disappointed Phibunsongkhram switched to courting the British and Americans to ward off what he saw as an imminent Japanese invasion. However, on 8 December 1941 the Japanese invaded Thailand at the same time as they invaded Malaya. Pearl Harbor was attacked in 'Ai' 90 minutes after the start of the Malayan and Thai invasions. Fighting between Japanese and Thai forces lasted only five hours before a ceasefire was agreed, and Thailand was allied with Japan until 1945.

After the war, in October 1946, north-western Cambodia and the two Laotian enclaves on the Thai side of the Mekong river were returned to French sovereignty when the French provisional government threatened to veto Thailand’s membership in the United Nations. This led to the conclusion of the Franco-Siamese Settlement Treaty of 1946 that settled the issue and paved the way to restoration of diplomatic relations between the two countries.

In the 'Franco-Thai War', The Vichy French army had suffered 321 casualties, of whom 15 were officers. The total number of missing after 28 January was 178 (six officers, 14 non-commissioned officers and 158 enlisted men). The Thais had captured 222 men (17 African, 80 French and 125 Indo-Chinese).

The Royal Thai army lost 54 men killed and 307 wounded. Some 41 sailors and marines of the Royal Thai navy were also killed, and 67 wounded. At the 'Battle of Ko Chang', 36 men were killed, of whom 20 belonged to Thonburi, 14 to Songkhla and two to Chonburi. The Royal Thai air force lost 13 men. The number of Thai military personnel captured by the French was just 21.

Some 30% of the Vichy French aircraft had been rendered unserviceable by the end of the war, some as a result of minor damage sustained in air raids and remained unrepaired. The Armée de l’Air admitted the loss of one F.221 and two MS.406 aircraft destroyed on the ground, but its losses were really greater.

In its first experience of combat, the Royal Thai air force claimed to have shot down five French aircraft and destroyed 17 on the ground, against the loss of three of its own in the air and another five to 10 destroyed in French air raids on Thai airfields.