Alternatively, this scenarios differences in the Siam-Indochina border may not make much difference to Siamese-Thai foreign policy right before and during Japan's launching of the Great Pacific War.
Sort of like in the last paragraph in the post above, Thailand may start to gravitate more toward Japan as it begins to project more power toward Thailand's neighborhood, and humiliates the French by forcing them to accept the occupation and supervision of French Tonkin. This, in addition to defeats in Europe, would have been dimming the prestige of France and Britain. Thailand may, like OTL, still have its border war over France, even though the size of its revanchist border claims on French Indochina is much smaller, no Laos, but some of the Central Highlands/Bolovens Plateau of Annam, and some border districts of Cambodia.
Thailand, like OTL, goes over to the Japanese after a mere 24 or 12 hours of resistance to Japanese advance/occupation. But throughout the war, it keeps some contacts with the Allies, and manages to convince the USA to restrain British desires for punitive treatment postwar [this all occur in OTL WWII and postwar].
The substantial divergences from OTL begin appearing only in the next war, the French Indochina War, which begins with isolated instances of fighting and skirmishing between British, liberated French, and repurposed surrendered Japanese troops in postwar British occupied southern Indochina in 1945 (south of the 16th parallel in Cochinchina, southern Annam, and Cambodia) against the Viet Minh, and later in northern Vietnam (north of the 16th parallel, in northern Annam and Tonkin) in 1946 between the better ensconced Viet Minh and redeployed French troops who arrive in mass to Tonkin's Red River delta upon the evacuation of Chinese Nationalist occupation troops.
The French command under Admiral D'Argenlieu in 1946 is determined to reestablish French colonial supremacy over Vietnam and Cambodia in its pre-war borders and essential pre-war characteristics. Negotiations from the French side were playing for time while positioning reinforced forces. The Viet Minh were trying to negotiate independence, however caveated and qualified, and to exert effective de facto political power, while cache-ing arms throughout the countryside and building up forces and informant networks in readiness for French attempts to completely crush their movement and reinstall the pre-war colonial regime.
The Viet Minh also provides support for a smaller, less significant, less ideologically sharply defined Free Khmer independence movement, providing material assistance, tactical and organizational advice, and ideological education. Since Laos is not a component of French Indochina, this timeline lacks the Lao Issara ("Free Lao") movement that OTL turned into the Pathe Lao movement closely aligned to the Viet Minh, DRV government, and VC/NLF.
The Laotian provinces are being increasingly included and integrated in Thailand's national infrastructure and development plans hatched in Bangkok for its northern interior in the decades after the war. The outbreak of the French Indochina War, and the Communist takeover of China only heightens concern in Bangkok which had existed since the encroachment of European colonial powers and the Japanese, about security of the northern interior frontiers.
France is unable to defeat the Viet Minh, but is able to hold strategic ground in Indochina through the late 1940s. The rise of the Communist government in China is a great boon for the Viet Minh, provided a secure source of steady cross-border aid and sancturary for the Viet Minh from late 1949 onward. However, the French are able to persist in their counterinsurgency effort, buoyed by a USA willing to dig deeper into its pockets to fund anticommunist struggles in Asia after the "loss" of China to Communism, and ready to increase support even more after the outbreak of the Korean War. France's overall recovery from WWII, aided by its own efforts, general European recovery, and Marshall Plan aid, also helps fund French forces in the field, and increasing numbers of noncommunist Vietnamese and Cambodia troops.
After the end of the Korean War in 1953, France is likely to tire of the Indochina struggle within a year or two. And whether or not its Indochina war wraps up, by some point in '54, '55, or '56, in all likelihood a rebellion in its Algerian department would likely emerge and command priority attention and resources and compel a conclusion of the Indochina effort.
One detail would necessarily be different about the French Indochina War endgame however. The climactic battle would not be fought at Dien Bien Phu, right at what is in this world, the Vietnamese/Indochinese - Thailand (Laotian provinces) border. While the Viet Minh at various points can get away with tactically and logistically "borrowing" Thai territory, they cannot count on setting up siege lines on Thai territory or risk emplacing precious artillery assets in Thailand where a concerted (although time-consuming and difficult and resource intensive) Thai campaign could undermine their position and its security.
It is likely that a battle similar to Dien Bien Phu is fought in some other valley within the "Viet Bac" region of northwest Tonkin, Vietnam, a little further east, or the French are simply induced to settle by overall, generic attrition via frontline combat along the perimeter of the Red River delta, guerrilla and terrorist attacks penetrating within the delta, other attack and attrition amounting to a siege and counter-siege of the central Annam cities of Hue and Da Nang and Quang Tri, fighting in the Annam Central Highlands, and ineradicable guerrilla warfare in the deepest interior portions of the Mekong Delta.
When a negotiated solution is made, the parties agree to temporary geographic regroupment/partition zones, with the Viet Minh securing the land north of the 17th or 18th parallel, and the French and their client monarchical "State of Vietnam" securing Vietnam south of that latitude line. The Viet Minh also evacuate their "Free Khmer" allies from Cambodia, leaving local independent governance to the French appointed Prince Norodom Sihanouk.
The concept in the peace accords is that "regroupment", involving mandatory movement of military forces, and voluntary movement of civilians, will be followed by all-Vietnam elections two years later.
The southern regime, ruled by Ngo Dinh Diem, after he ousts Emperor Bao Dai, refuses to hold all-Vietnam elections, alleging the northern election would not be free, and would yield a monolithic result dominating the whole control using the north's larger population numbers.
After the Franco-Viet Minh Accords ending the Indochina War, the US would also work to create a formal pact to contain Communist expansion in Southeast Asia, the Southeast Asia Treaty Organization (SEATO), including the USA, Britain, France, Australia, New Zealand, Pakistan, and importantly, among states local to the region, the Philippines and Thailand. While not extending a NATO article 5 commitment, the SEATO articles note it views the maintenance of peace and security in Indochina and its states, Vietnam and Cambodia, as a grave concern and cause for mutual concern and cooperation. There is no consensus among the SEATO allies however to extend an invitation to South Vietnam and Cambodia to join, because France and Britain read those states joining alliances as contravening the letter and spirit of the Franco-Viet Minh peace accords.
With the one additional potentially interested country, South Vietnam, being excluded for diplomatic and international legal reasons, no other countries the USA might have liked in SEATO are interested in SEATO membership. Burma and Indonesia are uninterested, indeed offended by the bloc's existence, and committed to a Nonaligned stance. Cambodia is also committed to neutrality and often employs nonaligned rhetoric. While Malaysia, and later, Singapore and Brunei, find themselves more attuned political and especially economically with the west, they see no reason to join the military alliance bloc.
Repression and rebellion, aligned with the Communists and given direction by them , flares up in late 1950s South Vietnam, and North Vietnam begins gproviding gradually increasing support from an initially low base, starting with redeployment of "regrouped" cadre from the south who had settled in the north after the armistice with the French.
In the early years, the VC/NLF insurgency escalates quite a bit in the early 1960s, and maximizes its use of southern manpower and resources, with additional cadre and materiel infiltrated in by land and sea. The higher the level of US aid to South Vietnam, especially introduction of large formations of US ground troops, ends up requiring infiltration of more men and materiel from the north to keep up the pace of operations.
A key differing factor in this TL's Vietnam War, or Second Indochina War, compared to OTL's is that there is either no Ho Chi Minh trail, or a much more limited one, running through the Laotian provinces of Thailand (that were simply Laos in OTL) and any North Vietnamese ground activity and infrastructure that rises to significant level will be constantly contested by Thailand army forces.
This could put a ceiling on the growth of infiltration trails and routes and movements of men (and vehicles) at a low level altogether, that could badly pinch the Communist ability to escalate, and then later sustain, high optempo fighting, once US forces do extensive operations to interdict maritime infiltration.
Communist Vietnamese coping mechanisms to this limitation could be to simply keep the conflict simmering at a lower level of intensity for longer, while awaiting cracks in US-South Vietnamese relations, or within South Vietnamese politics for opportunities to escalate military or political assertion, even though this means not having near-term victory as closely in sight.
Alternatively, it could lead to faster shift from rural guerrilla warfare to spectacular urban attacks to attempt to inspire general uprising, embarass the Americans, and wear away at US support for the US war effort. Pushing toward Tet harder and faster, with less. I would regard this as a riskier approach.
Or they could do something basically akin to OTL's approach, but try, possibly with Chinese, and perhaps Soviet assistance, to overcome and distract and shake Thai forces away from the border region by more generously supporting the armed guerrilla forces of the Thai Communist Party and operations it is attempting in any and all parts of that country. This might include, if politically and tactically useful, voicing support for Lao ethnic autonomy under Communism. The basic aim here would be to claw away Thai ground operations from the border area to allow the North Vietnamese to set up something like a Ho Chi Minh trail, to allow more robust infiltration and maneuver of North Vietnamese units into South Vietnam.
That itself is potentially vulnerable to Thai Communist proxies of any ethnicity not being up to tasks expected by Hanoi, or Thailand being resilient to the pressure and being capable of mobilizing to guard its frontier regions well with its own forces, or, as a last resort, inviting US forces operating in northern Vietnam to conduct joint operation in the Vietnam-Thailand border region against penetrating North Vietnamese forces and Thai Communist forces they are working with, at a time when it may still be politically viable within the US, for the US to do so. Cross-border American (and South Vietnamese) operations from Vietnam, at the invitation of an allied Thailand, would be non-controversial internationally, and much less controversial domestically, when compared with cross-border operations into a publicly, yet only fictionally, neutral country like Laos was in OTL.
There is also the chance that neutral Cambodia under Sihanouk may still choose OTL's version of "neutrality" and allow sanctuary for Communist Vietnamese forces on his territory and resupply for their forces through Sihanoukville port. But it is significantly less likely if prior US-South Vietnamese-Thai cooperative action is preventing North Vietnam from defiantly and effectively scaling up its escalations in response to US escalations in South Vietnam and along its borders.
