What if the Japanese overthrew the Vichy French administration in Indochina shortly after occupying it? I'm counting the occupation as the occupation of the whole, from July 1941, not the earlier, partial occupation, because acting in the earlier period would have made the Japanese peaceful occupation of the south impossible.
In OTL, the Japanese did not actually depose the Vichy French regime until March 1945, by which point they did it mercilessly, quickly, and efficiently.
What if they initiate the change in late August 1941, in a burst of radical anti-western enthusiasm, disarming, dispossessing, and imprisoning the French population at that time, and ensuring an ever stricter hold as they built up their forces in the territory then and through September and October 1941, and raised a puppet Vietnamese government, with auxiliary puppet Vietnamese troops, much like in Manchukuo, Mengjiang, and occupied China?
The Japanese approach here would be to support a puppet/client Vietnamese nationalism at French expense and at the expense of the local Chinese, Lao, and possibly Cambodian populations, promoting Japanese regional leadership above all, but Vietnamese supremacy above these other groups locally, to coopt Vietnamese cooperation and support for the occupation order.
Japan would still be extractive of the Vietnamese/Indochinese economy. Part of the purpose would be reducing "overhead" of French consumption - the Viets "expect less" to begin with , and ending threat of Vichy forces flipping to Free French or alignment with British or Americans, or the even more hypothetical threat of the Vichy French collaborating against Japan with an over-mighty globally victorious Nazi Germany. Pursuing this approach, also a builds a minor asset for pan-Asian, anti-colonial, disruptive, anti-western propaganda, and raising auxiliary puppet Vietnamese troops (as Japan had done in Manchukuo, Mengjiang, and China) to help with garrison duty, possible further campaigns against the western powers, and the southern front against KMT China and any local Communists that might spring up.
What impact does this have on the Indochinese, Sino-Japanese War, and regional Southeast Asian situation in the remaining months before Pearl Harbor? Western colonialists, including Americans not quite liking to be included in that category will be shocked and dismayed at the brutal treatment of the French population. I imagine the French would be imprisoned, with many killed and subject to abuses but largely put to labor under different forms of captive labor, or work-release under Japanese supervision depending on which of their particular skills the Japanese had use for. Also, the Japanese could try to charge Vichy France some 'ransoms' basically for return of some detained French people to other French possessions. Vichy France would be outraged, but would be militarily helpless to respond. Nazi Germany, Fascist Italy and their other European allies would feel obligated to make diplomatic complaints, publicly and privately, and not be entirely insincere. But they would not cancel the Tripartite Pact over it, nor cease trying to persuade the Japanese from entering the war with the USSR or any other Axis enemies.
Does the outrage cause the British or Dutch to do anything more effective with their defensive preparations? At the same time, does Japanese propaganda and pro-independence rhetoric, despite its hollowness, hypocrisy, and contradictions, inspire any additional native independence or anti-colonial agitation by Burmese, Indonesians, or Malays? Before anybody says "Of course not, they know what the Japanese did to Nanjing" - Please don't, Many didn't know, or didn't care, because they weren't Chinese. For all that people talk about preferring the devil they know, they often really do hate the devil they know more, and need to get to know the new devil in person or have people in *their* national group experience the new devil before they start to hate that one too.
I imagine many Vietnamese would sign up with the pro-collaboration Vietnamese government and forces for pay, the limited amount of situational power they get, and to not be a target. But the situation in no way dims the appeal of the Viet Minh movement with already established patriotic and class struggle bona fides, and leadership and fighting cadre hanging out in the Viet Bac and cross-border bases in South China. Vietnamese puppet troops can man local garrisons and, in some numbers, can accompany Japanese forces in their Thailand, Burma, and Malaya ops during the war, or in manning static defenses, sort of like the Hungarians did for the Germans. This won't make the Vietnamese a beloved people among the Allies or the Vietnamese independence cause very popular.
However, the Vichy French won't be there as intelligence contacts for the Allies either [they followed a two-faced policy of covert dealing with the Allies, assisting downed pilots when possible, alongside overt compliance/collaboration with the Japanese, while planning for eventual anti-Japanese uprising]. So, the only game in town for the Allies, the only people in Indochina they can assume won't turn in downed pilots or infiltrated agents, are Ho Chi Minh's Viet Minh guys.
How does this all play out through the end of the war and after? If puppet Vietnam has been assigned huge chunks of Laos and Cambodia by the Japanese, is Vietnam obliged by France, or just the UN in general, to yield back those territories postwar? Would Ho Chi Minh be cool with that? What about other flavors of postwar Vietnamese nationalists or monarchists?
In OTL, the Japanese did not actually depose the Vichy French regime until March 1945, by which point they did it mercilessly, quickly, and efficiently.
What if they initiate the change in late August 1941, in a burst of radical anti-western enthusiasm, disarming, dispossessing, and imprisoning the French population at that time, and ensuring an ever stricter hold as they built up their forces in the territory then and through September and October 1941, and raised a puppet Vietnamese government, with auxiliary puppet Vietnamese troops, much like in Manchukuo, Mengjiang, and occupied China?
The Japanese approach here would be to support a puppet/client Vietnamese nationalism at French expense and at the expense of the local Chinese, Lao, and possibly Cambodian populations, promoting Japanese regional leadership above all, but Vietnamese supremacy above these other groups locally, to coopt Vietnamese cooperation and support for the occupation order.
Japan would still be extractive of the Vietnamese/Indochinese economy. Part of the purpose would be reducing "overhead" of French consumption - the Viets "expect less" to begin with , and ending threat of Vichy forces flipping to Free French or alignment with British or Americans, or the even more hypothetical threat of the Vichy French collaborating against Japan with an over-mighty globally victorious Nazi Germany. Pursuing this approach, also a builds a minor asset for pan-Asian, anti-colonial, disruptive, anti-western propaganda, and raising auxiliary puppet Vietnamese troops (as Japan had done in Manchukuo, Mengjiang, and China) to help with garrison duty, possible further campaigns against the western powers, and the southern front against KMT China and any local Communists that might spring up.
What impact does this have on the Indochinese, Sino-Japanese War, and regional Southeast Asian situation in the remaining months before Pearl Harbor? Western colonialists, including Americans not quite liking to be included in that category will be shocked and dismayed at the brutal treatment of the French population. I imagine the French would be imprisoned, with many killed and subject to abuses but largely put to labor under different forms of captive labor, or work-release under Japanese supervision depending on which of their particular skills the Japanese had use for. Also, the Japanese could try to charge Vichy France some 'ransoms' basically for return of some detained French people to other French possessions. Vichy France would be outraged, but would be militarily helpless to respond. Nazi Germany, Fascist Italy and their other European allies would feel obligated to make diplomatic complaints, publicly and privately, and not be entirely insincere. But they would not cancel the Tripartite Pact over it, nor cease trying to persuade the Japanese from entering the war with the USSR or any other Axis enemies.
Does the outrage cause the British or Dutch to do anything more effective with their defensive preparations? At the same time, does Japanese propaganda and pro-independence rhetoric, despite its hollowness, hypocrisy, and contradictions, inspire any additional native independence or anti-colonial agitation by Burmese, Indonesians, or Malays? Before anybody says "Of course not, they know what the Japanese did to Nanjing" - Please don't, Many didn't know, or didn't care, because they weren't Chinese. For all that people talk about preferring the devil they know, they often really do hate the devil they know more, and need to get to know the new devil in person or have people in *their* national group experience the new devil before they start to hate that one too.
I imagine many Vietnamese would sign up with the pro-collaboration Vietnamese government and forces for pay, the limited amount of situational power they get, and to not be a target. But the situation in no way dims the appeal of the Viet Minh movement with already established patriotic and class struggle bona fides, and leadership and fighting cadre hanging out in the Viet Bac and cross-border bases in South China. Vietnamese puppet troops can man local garrisons and, in some numbers, can accompany Japanese forces in their Thailand, Burma, and Malaya ops during the war, or in manning static defenses, sort of like the Hungarians did for the Germans. This won't make the Vietnamese a beloved people among the Allies or the Vietnamese independence cause very popular.
However, the Vichy French won't be there as intelligence contacts for the Allies either [they followed a two-faced policy of covert dealing with the Allies, assisting downed pilots when possible, alongside overt compliance/collaboration with the Japanese, while planning for eventual anti-Japanese uprising]. So, the only game in town for the Allies, the only people in Indochina they can assume won't turn in downed pilots or infiltrated agents, are Ho Chi Minh's Viet Minh guys.
How does this all play out through the end of the war and after? If puppet Vietnam has been assigned huge chunks of Laos and Cambodia by the Japanese, is Vietnam obliged by France, or just the UN in general, to yield back those territories postwar? Would Ho Chi Minh be cool with that? What about other flavors of postwar Vietnamese nationalists or monarchists?