The Japanese deliver a shock

As France fell, the Japanese began to make demands of the Governor-General of Indo-China, General Catroux. When the General acceded to demands that rail traffic to China be stopped he was promptly replaced. Vichy named the loyal commander of the FNEO (Forces Navales d’Extreme-Orient), Vice-Admiral Jean Decoux, as Governor General. By September Decoux was facing far greater demands from the Japanese, including the right to station and transport troops through Indo-China, the use of selected airfields, and the evacuation of a hard-pressed Japanese division fighting in China through the port of Haiphong. An appeal to the Americans for help was poorly received.

Nationalist rebellions

The fall of Lang Son had almost immediate consequences for French rule. Discontented locals had witnessed how easily an Asian army defeated the whites. Vietnamese nationalist Tran Trung Lap was able to raise some 3,000 men in the Lang Son region, many of them deserters from the Indo-Chinese units defeated by the Japanese. Their arms were provided from French stocks captured by the Japanese. The returning French demonstrated they could still deal with a poorly trained rabble, and quickly drove the revolutionaries into the mountains, where planes and artillery hammered them. Tran Trung Lap was ambushed, and though he escaped the massacre of his men by machine-gun, he was shortly after captured and executed at Lang Son in December.

In the south of Vietnam, then known as Cochin China, an even more dangerous rebellion broke out in late November. Thai troops had begun to deploy along the Cambodian border and most of the garrisons in Cochin China had been sent to the frontier. Fighting broke out in the My Tho region and French police found themselves overwhelmed. The rebellion spread to Saigon and a number of southern provinces. A battalion of the Foreign Legion and a battalion of Tonkinese colonial troops on their way to Cambodia were diverted to the south and, with the help of artillery, air and naval detachments, quickly repressed the rebellion with utmost ruthlessness. The French had made their point, and could now send their forces west to deal with the Thais.

War with Thailand

The French now had to deal with a growth of militarism and Thai nationalism in neighbouring Thailand (the name was changed from Siam in 1938). Just as Germany sought to regain the territories lost in the Treaty of Versailles, Thailand was eager to retake the ethnic Thai lands along the Mekong River it was forced to cede to the French colony of Laos in 1904. In 1907 the French had also forced Siam to cede the largely Khmer provinces of Siemreap, Sisophon and Battambang to French Cambodia. The pro-Japanese government of Marshal Pibul Songgram sensed an exploitable weakness in the now isolated French colony, and began a military campaign to retake these territories after the French rejected demands for their return in October 1940.

The Thais had signed a non-aggression pact with the French in June 1940, but failed to ratify it after the collapse of metropolitan France. By October Marshal Songgram had mobilized 50,000 troops (in five divisions) and obtained 100 modern fighters, bombers and seaplanes from Japan. The Thai air-force was now three times the size of that available to the French, with the new aircraft added to the 100 American planes obtained between 1936 and 1938 (mostly Vough Corsairs and Curtiss Hawks). The Thai navy had also been equipped with modern ships and outclassed the French colonial fleet on paper at least. Border skirmishes began in November and the Thais crossed the Mekong in December. Hard-pressed elsewhere, the French could only commit fourteen battalions to the defence of Battambang Province.

On January 5, 1941, the Thais launched a full attack with artillery and aerial bombardment of French positions. The Thai offensive covered four fronts:

1) North Laos, where the Thais took the disputed territories with little opposition
2) South Laos, where the Thais crossed the Mekong by the 19th of January
3) The Dangreks Sector, where confused fighting went back and forth
4) Colonial Route 1 (RC 1) in Battambang province, where the heaviest fighting occurred.

The initial advance on the RC 1 was repulsed by the Cambodian Tirailleurs (riflemen). The main Thai column ran into a French counter-attack on January 16, colliding with the French at Yang Dam Koum in Battambang. The Thai force was equipped with Vickers 6-ton tanks while the French lacked any armour. The French counter-offensive had three parts:

1) A counter-attack on the RC 1 in the region of Yang Dam Koum
2) An assault by the Brigade d’Annam-Laos on the islands of the Mekong River
3) Operations by the naval ‘Groupement occasionnel’ against the Thai fleet in the Gulf of Siam

The main thrust of the offensive was by Col. Jacomy’s forces along the RC 1. The attack at Yang Dam Koum was a debacle from the start. The assault forces consisted of one battalion of Colonial Infantry (European) and two battalions of ‘Mixed Infantry’ (European and Indo-Chinese). The forest made artillery operations difficult, French aircraft never showed, leaving the skies to the Thai air-force, and radio communications were poor. The French transmitted orders using Morse code, perhaps explaining why the Thais often anticipated their movements. A complete rout was prevented when the Thais ran into a battalion of the Fifth regiment of Legion infantry at Phum Préau. The legionnaires were hit hard by a Thai armoured assault, but brought up two 25mm and one 75mm gun for use against the tanks. The motorized detachment of the 11th Regiment of Colonial Infantry reinforced the line, and three Thai tanks were destroyed, the rest deciding to retire. The diversionary assault on the Mekong was successful, but the largest battle of the war was to be fought in the Gulf of Siam.

Naval War in the Gulf of Siam

The French navy was all important in Indo-China, as with any overseas colony. The modest force had a virtually non-existent role in the great Asian war of 1941-45, being unable to resist either Japanese advances or Allied blockades, but they were nevertheless to have one great, unexpected battle before meeting an ignominious end. The fleet in Indo-China was divided into two parts with separate levels of responsibility. The FNEO was assigned responsibility for the overall defence of French colonies in Indo-China and the Pacific, while the Marine Indochine with its river gunboats was responsible for interior security in Indo-China.

The Donburi was spotted attempting to escape through the 200m high islands and the French cruiser set off in pursuit. The Donburi was set afire but continued to engage the cruiser and the sloops, which now began to pour fire into the Donburi. Badly damaged and listing to starboard, the Donburi eventually disappeared behind an island and the French broke off. Later in the day the Donburi was taken in tow by a Thai transport but capsized soon after. Throughout the engagement the French sailors were impressed by the courage of the Thai sailors under fire.

The French ships were unable to exploit their victory, however, due to the arrival of Thai Corsairs targeting the Lamotte-Piquet. Fierce anti-aircraft fire drove off the attacks and by 9:40 AM the French turned for home. In a brief but decisive engagement the Thai fleet had been destroyed at negligible cost to the French. It appeared at the time to be a sudden and dramatic reversal of French fortunes.

Aftermath

The Japanese had seen enough and accompanied an offer to mediate the conflict with the arrival of a powerful naval force off the mouth of the Mekong River to encourage negotiations. A tentative armistice was imposed on January 28, but Thai provocations on the frontier continued until a formal armistice was signed aboard the Japanese battleship Natori off Saigon. The extent of Thai-Japanese collaboration was revealed when a Japanese-imposed treaty between Vichy and Thailand was signed on May 9, 1941. The disputed territories of Laos, part of the Cambodian province of Siem Réap and the whole of Battambang were awarded to Thailand. The conflict had cost the French over 300 men and a further loss of prestige amongst its colonial subjects. European troops and material losses could not be replaced due to the blockade. The French garrison remained highly demoralized until the Japanese coup in 1945 destroyed the Vichy colonial army in Indo-China.

In the end the Thais fared little better. The Khmers largely evacuated the lost Cambodian territories, preferring French rule, and Thailand itself was soon occupied by its more powerful ally, the Japanese. American Flying Fortresses bombed Bangkok in 1942. The Thais declared war against the allies in 1944, but there was some confusion over whether the declaration was actually delivered to the US government, and after the war the Thai government certified the declaration of war as null and void. The uncomfortable affair was mutually forgotten. The disputed territories in Laos and Cambodia were returned to the new Gaullist government at the end of the war.