The Franco-Thai Border War 1940-41


A protest march in Bangkok demanding the return of territory from France.

If it is correct that one of the first casualties of war is truth, then certain previously accepted ‘facts’ about the Franco-Thai War of 1940-1941 need to be re-examined with greater scrutiny.

The Thai accounts of that brief war and the French versions tend to differ, sometimes by degrees and occasionally quite markedly. This is particularly so with regard to the causes of the war and especially the naval battle of Koh Chang.

France and Siam had been on the verge of war on a number of occasions from the mid-nineteenth century onwards, as the French sought (successfully) to extend their empire in Indochina and the Thais struggled in vain to hold on to the vestiges of control they exerted over their ostensible vassals of Laos and Cambodia.

Following the First World War, Siam hired German advisers to help build and train a Thai army. By 1935, Siam had a modern army of some 50,000 troops and it was in this year that yet another border incident took place between the Thais and the French.

A Thai-run logging company, owned by Khun Inta Bangcongcit, would purchase logs in French-controlled Laos and float them down the Mekong River to Siam. However, the French allegedly broke the logging agreement, causing Khun Inta to lodge a complaint with the French authorities. They responded by arresting and beating Khun Inta and allegedly raping his wife. This prompted Siam to make an official complaint to the League of Nations. France was ordered to pay compensation to Khun Inta and his wife, but the French ignored the order.

This complaint was the catalyst that led to Siam re-negotiating a series of treaties regarding navigation, commerce and extraterritoriality with 13 nations, including Britain, France, the United States, and Japan.

In 1938, Siam approached France and asked to renegotiate their common boundaries to prevent incidents similar to those of Khun Inta occurring in the future. The French refused to enter into any form of negotiation.

Following the fall of France to the Germans in June 1940, the Thais approached the new Vichy-aligned government in Indochina with regard to settling their border difficulties, believing they would be more amenable than their predecessors. The Thais were sadly mistaken, but the British, concerned about the very real possibility Indochina would fall to the Japanese, put pressure on the French to enter negotiations.

In August 1940, Japan, aiming to improve their strategic position against China, demanded the Vichy government in Indochina give them unfettered use of three northern Vietnamese airfields as well as allowing 5,000 Japanese troops to be stationed in the Red River Valley.

The French colonial administration agreed in principle to the demands, but haggled over the details. However, by September 1940, the Japanese were effectively in control of northern Vietnam.

Prime Minister Pibul Songgram offered to sign a Non-Aggression Pact with France, similar to the one then in force with Japan and Britain. Talks, led on the French side by Admiral Decoux, the Governor-General of Indochina, commenced early in September 1940 with six major points to be addressed between Thailand and France. These involved clear demarcation of certain border areas as well as the return of some territory to Thailand. The negotiations lasted just two weeks before they broke down through the intransigence of both sides.

In retaliation, the colonial French authorities began making life difficult for Thais doing cross-border business. A number were arrested and in October, matters were exacerbated when Khun Canta Sintharako, after crossing into Cambodia to visit his business partner, was beaten to death by police in front of his family.

The Thai government demanded an investigation, a formal apology and a renewal of border negotiations. The French reply, according to the Thai version of events, came in the form of provocation with military aircraft over-flying Thai territory and artillery shelling border posts. French patrols started making incursions into Thailand.

Thailand responded by sending aircraft to patrol the border with orders to attack any foreign troops they encountered on Thai soil. The army also moved artillery units up to the border and returned French fire.

The French claimed the provocation came from the Thai side, with units being sent across the Cambodian border in probing actions.


Painting of a 1941 bombing mission undertaken by Thai military aircraft against the Vichy French. (from a painting in the Royal Thai Air Force Museum)

Whichever nation was at fault, by November, border incidents were becoming more common and serious. The French, with around 100 military aircraft available, sent their bombers in at night to attack military positions and border towns, avoiding Thai aircraft. The Thai air force, according to League of Nations documents of the time, numbered around 207 planes (of which only 128 were in tactical units and considered modern) and some of these allegedly attacked places such as Battambang and Vientiane. The French claimed to have shot down four Thai aircraft in dogfights, though the claims are disputed.

On 1 December 1940, the undeclared war escalated when the French navy sent three sloops to shell the coastal town of Trat. Three Thai planes attacked the sloops, hitting one. The French then shelled the border town of Aranyaprathet, killing six civilians. A French assault against Nakhon Thanom the following night resulted in the deaths of two more civilians.

The Thais lost four planes to French action from 9-13 December, three were shot down and the fourth destroyed on the ground when French aircraft bombed Ubon. The tale of woe for the Thais continued when two aircraft collided on take-off from Ubon on 14 December.

In an attempt to stop French aerial attacks, six Thai bombers struck Ban Sin airbase on 16 December, damaging the field and wrecking a few French airplanes.

During the Christmas-New Year period there was a lull in the fighting. However, on the night of 4 January 1941, French bombers struck Udon Thani and Nong Khai. Five days later, Thai aircraft conducted a series of daylight raids against Battambang, Sisophon, Vientiane, and Pakse among others.

On 10 January, French reconnaissance planes were sighted over Bangkok. The Thai capital was also subjected to an innocuous bombing raid that same day. At the same time, three Thai battalions crossed into Laos, met negligible opposition and in succeeding days occupied the entire western bank of the Mekong north and south of Paklay.

Also on 10 January, the largest of the Thai assault forces, consisting of nine battalions, two artillery regiments and tanks, struck Poipet before moving in the direction of Sisophon.

In a somewhat daring action, while French planes were targeting Bangkok, six Thai bombers and four fighters attacked Hanoi. Four French fighters attacked them, but the Thais claimed to have shot down two French planes, a claim refuted by the defenders. It is known that two French planes were out of action at the end of the war, possibly damaged by the Thai attack and it is almost certain one of them was shot down.

On 11 January, Thai planes attempted to knock out the main French air base near Siem Reap. The French claimed to have shot down at least five Thai aircraft (a claim disputed by the Thais) while the Thais believed they destroyed two French fighters in dogfights as well as another on the ground. The French did not confirm these claims. The Thais admitted to losing one fighter, the last of seven planes they lost during the war.

Thai forces attacked towards Pakse on 12 January and within a week had gained control of the region as far south as the Cambodian border.


Royal Thai Air Force fighters engaged in an aerial battle with the French. (from a painting in the Royal Thai Air Force Museum)

In northern Cambodia, three Thai battalions pushed past Samrong, which they burned, before being halted by determined French defence around the end of January. Among the combatants was Prem Tinsulanonda, a future Thai prime minister.

The French were hampered in their defence of both Cambodia and Laos by a revolt in the Mekong Delta region of southern Vietnam that tied up forces they could have used against the Thais.

Two prongs of the Thai offensive moved towards a junction at the Tonle Sap while the third section struck towards Battambang. The French, thinking the strike towards Battambang was the main Thai thrust, moved the bulk of their forces to counteract the drive.

The folly of this thinking was soon exposed when the other two Thai prongs swept aside resistance, surrounding the main French army. The Thais planned to wipe out the French, drive on and occupy Saigon and compel France to restore the territories it had taken in the previous 55 years.

Admiral Decoux had, in early December 1940, put together a small naval squadron called the Groupe Occasionnel in Cam Ranh Bay, placing Captain de Vaisseau Berenger in command.

In an effort to relieve the pressure on the army, the ramshackle squadron sailed into the Gulf of Thailand on 13 January 1941 to seek out their Thai counterparts. It appeared a David and Goliath contest as the Thai navy had been upgraded with ships purchased from Italy and Japan and the French had no real air cover apart from eight seaplanes based at Ream to provide reconnaissance.

On 14 January 1941, the naval battle of Koh Chang took place. The details of the battle, an undisputed victory for the French, are controversial, with both sides offering almost diametrically opposed versions. The French claimed to have sunk or destroyed no less than five Thai warships; the Thais allege they lost only two.

The French squadron consisted of the light cruiser Lamotte-Picquet (the flagship commanded by Captain Berenger) and four sloops, two of them quite ancient.


The French flagship, Lamotte-Picquet.

The battle began when a French seaplane found a section of the Thai fleet, consisting of two coast defence ships (HTMS Ayutthaya and HTMS Thonburi, both commissioned between 1937 and 1938), two sloops, four torpedo boats, two minesweepers and a patrol boat, anchored off Koh Chang and attacked them.

Berenger then led his ships against the Thai vessels in an action that commenced near dawn and lasted around an hour before the French withdrew- according to them- unscathed.

The French alleged their flagship struck the Ayutthaya with a torpedo and, heavily damaged by gunfire, she was beached by her crew on the mainland. The Lamotte-Picquet severely damaged a Thai torpedo boat, which later sank, before sinking two other torpedo boats.

Berenger’s squadron then turned all their guns against the Thonburi and claimed to sink the vessel.

The squadron, its mission accomplished, began sailing back to the relative safety of French-controlled southern Vietnam but came under aerial attack, which they managed to beat off with anti-aircraft fire.

The Thai version of events is somewhat different. The action opened with the French concentrating their initial salvoes on the two Thai torpedo boats anchored near the approach to Koh Chang. These two vessels were overwhelmed by gunfire and sank.

The Thonburi began returning fire some 15 minutes after the attack commenced while the remainder of the fleet, caught by surprise, started raising steam with the aim of sailing out to battle.

The Thonburi, struck by French shells, began to burn and, suffering from serious flooding, was beached. However, the remainder of the Thai squadron started engaging the French.

The Ayutthaya fired on the French cruiser Lamotte-Picquet while two Thai torpedo boats engaged two of the French sloops. The Ayutthaya claimed to have scored a direct hit on the French flagship, penetrating the engine room. An American reporter who saw th ship back in Saigon later said he could see no evidence of damage.

Faced with a Thai force now ready to fight at closer quarters and with his flagship hit, the French commander broke off the action and withdrew. The Thais lost 36 men killed in the action. French losses are unknown.

Following the battle, the crew of the Thonburi put out the fires on their ship and re-floated her. She was towed back to Bangkok where she entered dry dock for repairs. The ship eventually became an accommodation hulk in Bangkok before being scrapped in 1956.


HTMS Songhkla on fire during the Battle of Koh Chang.

In reality, the Ayutthaya received only superficial damage and was eventually sunk during the abortive coup of 1951. Two of the torpedo boats allegedly sunk by the French in fact remained in service until 1976.

Nevertheless, the French victory was their first against an opponent of equal strength since 1781, when they defeated the British in the second Battle of the Virginia Capes during the American War of Independence.

The final major action of the war took place either on 16 or 22 January (depending on whose account you read) when Thai units, supported by tanks, encountered a French defensive line at Yang Dang Khum, northwest of Sisophon. The French claimed to have held their line and forced the Thais to retreat, while the Thais stated they were waiting to move sufficient forces into the area to complete the destruction of the French.

The French also claimed to be preparing for a major counter-attack against the advancing Thai army and stated the Thai army had suffered around 800 casualties against 120 of their own. These claims seem exaggerated, although the Thai advance did stall.

On the other hand, the Thais state they had almost surrounded the remainder of the French forces defending Cambodia and were within three to four days of forcing the French to surrender.

While the fighting was raging, behind the scenes Japan offered to act as a mediator, encouraged by Prime Minister Pibul Songgram. With both sides accepting Japanese mediation, a ceasefire was arranged for 28 January. An armistice was signed three days later, on board the Japanese cruiser Natori in Saigon harbour, bringing military action to an end while a peace agreement could be hammered out.

The casualty figures for the war vary enormously; depending on which source is being referred to. Overall, deaths on both sides were at least 700 each on both sides, while one estimate goes as high as 2,000 military and 2,000 civilian casualties. Of the latter, two Catholic nuns and five catechists (one man and four women) were killed during what the Catholic Church deemed as persecution of Christians, Catholics in particular. The Thai government made it known they considered Catholic Thais to be traitors to their country for sharing a faith practised by so many French. The Catholic Church beatified the seven ‘martyrs’ as they termed them in 2000.

A large number of French Foreign Legion troops, taken prisoner during the invasion of Laos by the Thais, refused to be repatriated at the end of the war and decided to settle down in Thailand.

On 7 February 1941, a peace conference between the Vichy French government of Indochina and Thailand began under the auspices of the Japanese Foreign Minister Matsuoka Yosuke in Tokyo.

The Thai delegation, led by respected career diplomat Prince Wan, commenced negotiations with a series of ambit claims that the French, headed by Rene Robin and the ambassador to Japan, Paul Arsene Henry, could not possibly accept.

The two sides were deadlocked until the Japanese Foreign Minister presented a set of proposals some 10 days after negotiations had started. Rene Robin countered with a set of alternative proposals and, despite the fact that Prince Wan felt the Japanese were biased towards the French, he was authorised to accept the terms after speaking with Prime Minister Pibul Songgram.

On 11 March, a mediation agreement was signed in Tokyo while on 9 May the Treaty of Tokyo officially ended the war.

By the terms of the treaty, the French were forced to cede the Cambodian provinces of Siem Reap (although not the temples at Angkor Wat) and Battambang to Thailand as well as Sayaboury in Laos. Thailand agreed that the ceded areas would be turned into demilitarised zones. In return, Thailand agreed to pay the Vichy French government the equivalent of 10 million baht in compensation.

In turn, the French signed a treaty with Luang Prabang in August 1941 ceding Vientiane, Xieng-Khouang and Luang Nam Tha to the French protectorate to compensate it for the loss of Sayaboury.

Japan exchanged letters with Thailand and France stating the agreement was irrevocable as long as neither country entered into any pact with a third power which could be deemed hostile to the Japanese.

To celebrate their success in the war, Pibul Songgram erected the Victory Monument in central Bangkok in late 1941.