From Singhsong: The Japanese decide not to mount their invasion of French Indochina in September 1940, the main objective of which was to prevent China from importing arms and fuel through French Indochina (with the Vichy French government asking the Nazi German government to intervene, to moderate the demands of its Japanese ally, as IOTL; but whereas the Nazi Germans sat back and refused to do anything IOTL, they do ITTL). And as such, Japan decides to conduct the campaign against French Indochina entirely through its proxy instead; encouraging the Kingdom of Thailand's Phibun regime to proceed with its planned invasion in October 1940, and initiate the Franco-Thai War as IOTL.
ITTL, though, unlike IOTL, Imperial Japan (irked by Nazi Germany's threats to intervene on Vichy France's behalf) refuses to step in to subsequently mediate the conflict, negotiate a general ceasefire, or sponsor any sort of "Conference for the Cessation of Hostilities"- willing to sacrifice their working relationship with Vichy France and the status quo, in order to cut off their continued supplies to the Chinese, and bring the region under their indirect influence as part of their newly-established Greater East Asia Co-Prosperity Sphere. As such, with the Japanese also choosing to provide significantly greater support to the Royal Thai Army and Navy than IOTL (and with the French losing the Battle of Ko Chang, when the bomb which scored a direct hit on the Lamotte-Picquet IOTL actually explodes ITTL), the majority of French Indo-China falls under the occupation of the Kingdom of Thailand instead; with a ceasefire still reached without Japanese arbitration, but with France forced to suffer the ignominy of ceding Cambodia and Laos in their entirety to Thailand in the peace treaty signed in Bangkok.
In return for standing aside and allowing Thailand to greatly expand its borders, the Japanese seek to use Thailand and Indochina as their military bases to invade British Burma and British Malaya later, and win a secret verbal promise from Phibun to support them in an attack on Malaya and Burma. However, greatly emboldened and boosted in popularity by the far greater success and prestige of Thailand in the aftermath of TTL's Franco-Thai War, and with Thailand's industrial and economic base having also being greatly boosted by the annexations of Cambodia and Laos (as well as by the subsequent uprising of not just the Viet Minh, but a revived and more powerful Vietnamese Revolutionary Army, which obtains the full support and backing of the Kingdom of Thailand, rather than having to seek out support in Yunnan and being brought under the umbrella of KMT activities, as it was IOTL with Chinese KMT General Zhang Fakui's creation of the 'Vietnam Liberation League') Phibun reneges on this agreement (as he did IOTL).
Relations between Japan and Thailand subsequently become stressed (even more so than IOTL), and with the increasing threat of an imminent invasion by the Japanese becoming readily apparent, a disappointed Phibun chooses to compromise with the anti-Japanese, left-leaning leader of his civilian government, Pridi; with the Thai government seeking to adopt a policy of neutrality, and asserting that Thailand should remain neutral, only going to war to defend its sovereignty against foreign invaders. And with both the British and the Americans willing to invest more effort into courting the Thais themselves ITTL, with Thailand actually having become the 'Oriental Italy' ITTL, Phibun meets the Japanese threats of invasion by threatening to join the Allies, in the event of a Japanese invasion of Thailand (or of French Indochina, which is in open revolt by the spring of 1941, with TTL's 'Vietnam Liberation League' actually being a Vietnamese nationalist revolutionary movement seeking full independence, and successfully ousting the Vichy French regime to achieve this by the end of the year).
And as such, the Japanese never invade Thailand nor Indo-China ITTL. Thailand still joins the war on the side of the Axis, as IOTL. But after the announcement of the planned expansion of the Japanese Greater East Asia Co-Prosperity Sphere, which Phibun's even more offended by ITTL than he did IOTL- seething at the sheer arrogance of the Japanese, in still deeming him and his government to be a mere puppet of theirs, only on the same level as Wang Jingwei and the governor of Manchukuo- he puts his foot down, and not only outright refuses to attend the following year's Greater East Asia Conference, but refuses to even dispatch an envoy on behalf of the Thai government (with Thailand thus never becoming a member of the Sphere ITTL). Instead, Phibun pursues his own vision of the "Great Thai Kingdom policy", akin to Mussolini's vision of "Mare Nostrum"- albeit persuaded to water this down slightly by endorsing and acknowledging the independence of Vietnam (albeit as a puppet state of Thailand, under Thai occupation, inspired by the Japanese and Italian examples of Manchukuo, Croatia and Greece).
This irrevocably damages Imperial Japanese-Thai relations (though not enough to initiate hostilities between them). But Phibun's efforts to distance himself from the Japanese pick up pace with the Allied Invasion of Italy and the downfall of Benito Mussolini, sending shock waves through the Thai government. An emergency cabinet meeting is convened to discuss the European situation, and shortly after the Kingdom of Italy officially joins the Allied Powers and declared war on its former Axis partner Germany, with the Empire of Japan having also already been forced back onto the defensive by this point in the war, the Kingdom of Thailand (and its installed puppet regime in Vietnam) also follows suit in officially joining the Allied Powers and declaring war on its former Axis partner Japan- which by this stage is too beleaguered to retaliate by mounting an invasion of the former territories of French Indochina.
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