From Storm Clouds over the Pacific, 1931–1941, by Peter Harmsen (War in the Far East, Book 1; Casemate, 2018), Kindle pp. 217-219:
In early January, forces of the Kingdom of Thailand crossed the border into French Indochina in four different sectors from northern Laos to Cambodia. The attackers made swift progress in most places. Pockets of resistance were wiped out by over-whelming firepower. At the southern edge of the Thai advance, scattered fighting took place along the Route Coloniale 1, the main road connecting Bangkok to Phnom Penh and the other major cities of French Indochina. The French defenses, made up to a large extent of Indochinese recruits, considered the terrain near the road unsuitable for defense and pulled back, allowing the Thai forces to occupy large tracts of land virtually unopposed.
The Thai offensive came as no major surprise to the French. Thailand, one of few Asian nations to escape Western colonialism, had been tempted by the speedy defeat of France in the summer of 1940 to request the return of territory in Laos and Cambodia that had been ceded to the French colonial power in the preceding decades. Part of the Thai motivation was also a desire to act fast and seek a strengthened position in this particular part of Asia before Japan moved in and made it impossible. Following the political fashion of the 1940s, Thailand carried out the drive for more land in the name of bringing “all Thai people” under one government, even though not all the areas claimed by Bangkok were inhabited by people that could justifiably be described as Thai.
In addition, there were domestic reasons for Thailand’s sudden aggressive demeanor. Militarism was growing in the country, and the civilian leadership was increasingly dominated, or rather threatened, by the Army’s jingoistic top brass. Early in the crisis with France, while the United States was seeking to mediate, Washington’s ambassador to Bangkok was visiting Thai Prime Minister Pibul Songgram at his private residence. The American envoy noticed that Army officers were sitting in an adjoining room, listening in on the conversation through an open door. “They might kill me if I do not follow their desires,” the Thai prime minister told his American visitor.
The mediation made little difference, and by late 1940 tensions between France and Thailand had built up. In December, all Thai nationals had left French Indochina, and in the end the diplomatic staff at the Thai consulate in Saigon had been ordered to pack up and sail for Bangkok. In the same month, Thai airplanes dropped bombs over the French colonial city of Vientiane. French pilots who were scrambled to intercept the bombers were surprised to be faced with aircraft that were “extremely well flown.” It seemed, they said, that the Thai pilots had “plenty of war experience.”
Once the land invasion in early January 1941 was a reality, the French military commanders in Indochina set in motion contingency plans prepared a few months earlier. It called for the concentration of the few forces available in a two-pronged counterattack in the forested area around Route Coloniale 1 on January 16.