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.
This is super-creative, and I love it, even if I have no idea if the Thai, or native Viet forces could be that strong against the Vichy garrison and administration! This scenario never would have occurred to me.
A couple questions on your scenario -
the majority of French Indo-China falls under the occupation of the Kingdom of Thailand instead
The Thai occupy the majority of it, including parts of Vietnam? They wash their boots at Saigon or Da Nang or Qui Nohn? They don't just recover all of their former Laotian and Camboadian tributaries? That is quite impressive. That doesn't just reverse the verdict of the 1893 war, but of the last couple centuries wars. Or by 'majority' do you just mean Laos and Cambodia? They together are more land area than the three parts of Vietnam, but of *far* far lesser population, lesser cultivated land.
(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
OK, from spring 1941 on, Vietnam is in uprising, with the French not ousted instantly, but in danger. There are two main revolutionary contenders, the Viet Minh, a Communist-led United Front organization mostly following COMINTERN organizational models, and maybe some cash, but not any real Soviet/ChiCom arms pipeline, and the VRA, which in this timeline gets mainly Royal Thai patronage instead of Chinese patronage.The French I imagine are vulnerable to the uprising because of material and especially prestige losses from the Franco-Thai war.
Phibun reneges on this agreement [with Japan against the Allies] (as he did IOTL).
OK, I guess I can see it, even after he got more generous support from the Japanese, which probably allowed the Japanese to make and bribe more contacts in Thailand and collect more intelligence for covert ops and invasion than they had in OTL.
It's tricky to see how Thailand can extend its deterrence effectively beyond its own Laos and Cambodia protectorates to include also Vietnam, which between the spring 1941 and Dec 1941 is the scene of decisive struggle by the VRA and Viet Minh against the Vichy French garrison administration. The messy 'Vietnam War' or 'Annamite' or 'Tonkinese' war seems like a situation that would be hard to exclude the Japanese from getting into - If for no other reason than any badly losing faction on the Vietnamese or especially French side could be likely to invite the Japanese in to referee and rescue them. (Here's a wildcard for ya, after the April 1941 Japanese-Soviet neutrality pact, and with Japanese relations with Thailand and its allies tensing, and the Vichy still angry at past Japanese demands and hostility, the comparatively isolated Japanese and Viet Minh decide it is kosher for them to work together cooperatively against their mutual French, Thai, VRA, and Chinese enemies!).
But I guess your argument is the Thai, both Phibun and Pridi, are able to make their 'keep out of our countries, and keep out of Vietnam too' threat convincing by threatening to join the Allies [at this point just Britain with silent American backing, and probable cooperation with China to the north] if Japan makes any move on mainland Southeast Asia.
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).
Petain spoke for all Frenchman, even his archenemy Charles De Gaulle, when he said on December 8th after Vietnamese rebels stormed the final French bastions of Saigon, Dalat, Haiphong, and Danang:
"Oui, le 7 décembre, 1941, un jour qui vivra dans l'infamie, l'œuvre de générations de fils de France est tombée en ruine, alors que des bandits radicaux incultes ont exploité la crise mondiale pour brûler la mission civilisatrice de la France en Extrême-Orient !"
traduction en anglais:
"Yes, on December 7th, 1941, a day which will live in infamy, the work of of generations of the sons of France fell to ruin, as radical miseducated bandits exploited the global crisis to burn down the civilizing mission of France in the Far East!"
But- it would still be interesting to see more detail on how the Vietnamese rebellion/war effort works out, whether the VRA/VLL and Viet Minh form a coalition of convenience, and which leadership ends up stronger in the aftermath of beating the French and in the years down the road. Also the details of how the French surrender, evacuate, or get eliminated to the last man since the global situation is not conducive to any form of organized withdrawal.
And as such, the Japanese never invade Thailand nor Indo-China ITTL.
This is the important part where you meet the requirements of the AHC.
Thailand still joins the war on the side of the Axis, as IOTL.
But we have skipped over the important intermediate steps like: 1) Why has Japan started the greater Pacific War, 2) Has Japan been embargoed an why, without its Indochina occupation, and 3) Why is Thailand not just sticking with the neutrality it struggled for, and instead joining the Axis?
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 brings up an issue - will the Vietnamese, a people possibly more numerous than the Thai, and thinking themselves more sophisticated, going to be contented as puppets of Thailand. Thailand and Vietnam had been historic rivals and had fought each other *more* recently (in the 1800s) than China and Vietnam fought, and Thailand lacked the cultural gravity and prestige that China or even Japan had.
This irrevocably damages Imperial Japanese-Thai relations (though not enough to initiate hostilities between them).
This brings up an important question - in its time as an axis power, is Thailand, on its own, or by hosting Japanese forces, invading Burma and Malaya? That can be hard for Britain to forgive. What happens to Burma, and the Burma Road, makes a big difference to China.
The ending of your scenario, and Thailand flipping back to Allied makes sense as a culmination of the trends you had going beforehand.
