Thus, for the scenario I was considering preventing the invasion of Manchuria (leading to further tension between Japan and US, UK and the former's isolation from the League of Nations) because after that it is harder to prevent war from escalating.
Dissuading Japan from incorporating Manchuria is interesting; I would have to read up on exactly how that started and why, but I suspect I will find it is in the early '30s. It is my belief, which might or might not stand up to more detailed knowledge, that basically Japan was weakly and nominally liberal, but in fact a very traditionalist-authoritarian society, up to the Depression, and then the Depression hit them hard. Japan has few resources of any kind in the Home Islands; holding Korea, Taiwan and the outer islands scattered in the Pacific, many of which they picked up from Germany during the Great War as an Ally (of Britain) would augment them but little. Korea was probably most important for general resources and that presumably is why they widened their claims to Manchuria, hoping for more as well as Chinese workers to subjugate.
Basically, between no domestic resources to speak of and being a latecomer to the world capitalist system, the only non-European ancestrally power to manage to do so on its own terms, they had little leverage for honest trade in a world sewn up into various imperial spheres of interest. Prior to the Depression, the various imperial holdings did trade with each other enough for Japan to get into niches, but when the global markets crashed the general solution for their own domestic interests the various relevant European imperial nations, and the USA, pursued was "imperial preference" as the British called it, earmarking resources and hoarding markets for maximum opportunity for their own home nation, followed by keeping the colonies more or less on life support as seemed affordable, with any foreign interests running a distant third priority. No one would prioritize trade with Japan, so the Japanese were left with ability to make products of fair quality but no one interested in buying, and without selling something, they could not even have the resources to make anything.
Left high and dry by the market system, what liberalism there was in Japan was eclipsed and militarists who decided the thing to do was seize resource areas for Japan and produce for Japan took the lead. This I think then was the hard drive leading to adventures in Manchuria, then the rest of China, and hence to the Pacific Theater of WWII.
What is to be done here? Japan does not have the resources, that's what they needed. A reliable trade partner might enable more moderate leadership to prevent the more fanatically militaristic factions from taking power, but who would do this?
I have several vague suggestions, radiating backward in time for earlier PODs, each unfortunately of dubious plausibility:
1) latest, and playing off your own quote above, if I read you right you would have both the USA and UK leaning hard on Japan to block their conquest of Manchuria by brute force, or threat of it. I don't think a simple US/UK united front would work, nor is it at all probable to happen.
But maybe, a good cop/bad cop dynamic might work? The USN had concluded after the Great War that Japan was the obvious foe to prepare for war against, in the Pacific anyway. Between domestic US racism and US ambitions to dominate the Pacific that made Japan our rival, the Americans could hardly be expected to be reasonable with Japan.
But Britain on the other hand had cultivated a Japanese alliance for some time, which paid off quite well for Japan in the Great War when basically the British delegated to Japan the business of taking out the German colonies in the Pacific for the reward of getting to keep them. Prior to that, Japan's cordial relations with Britain had paid off several times. What Britain got out of it, I have not seen thorough studies of, but it was plainly of some use to Britain not to have to base extensive fleet or other military assets in the distant Pacific when the foes they expected to have to fight were like themselves centered in Europe. And Britain and her empire were major trade partners with Japan, I expect. So it was a bit of a dissonance for the British to find the American allies they figured they needed were at odds with the Japanese allies they had been getting along with quite well.
Meanwhile, will the USA take any position opposing Japan in Manchuria, when all that is happening so far away and the US has other problems?
I suspect not with Herbert Hoover being President. But suppose we had some other President, presumably a Republican, but one who had stronger ties to the China lobby, and gave more priority to the USN brass's opinions, and maybe perhaps was not afraid to saber rattle thinking a possible war might distract Americans from their miseries in the Depression and might even kickstart the economy again? This sort of opportunistic militarism would be a break from OTL 1920s Republican"return to normalcy/business of America is business" isolationist mentality, but perhaps we can account for it with suitable examination of personalities in US politics in 1928--before the Crash. I think I am describing someone not unlike William Randoph Hearst here--who might not be suitable to be elected himself, but might perhaps ally with someone else who is?
Meanwhile we might need to also butterfly whoever is Prime Minister in Britain. Perhaps the American faction of the Rs (probably with an agenda like this, enjoying some support from some Southern Democrats too) that is in ATL ascendency is also somewhat prone to Anglophobia, a not too uncommon position of some Americans, particularly some Republican supporters, in the post-Great War period where the British were accused of tricking Americans into the war to pull Entente chestnuts out of the fire. The French were little blamed for this, but the English were.
So meanwhile someone other than the OTL Tory PM, Baldwin IIRC. Perhaps a Liberal gets elected?
Now perhaps this ATL early 1930s PM values the Japanese alliance--as late as 1941, Churhill claimed in his post-war memoirs on the war period, he lamented the sad fact that Japan could not be concilated as the Americans would veto it.
But this is 1932, not '41. Hitler has not taken over Germany yet; the Soviet Union seemed well contained and if it came to war with them, the Japanese would be the obvious allies to want to keep the Russians occupied on the Pacific. Italy is not a major threat and France could contain them pretty well. The League of Nations might seem well able, early in the decade, to keep peace in general in Europe, largely following more or less common-interest British and French lead. The British recognize that the USA would be a terrible foe to have, in part because much of British capital was in fact invested in the USA. But at the moment Britain does not seem in need of Yankee patronage, whereas the Yankees are being boorish about poor Japan.
So I am imagining the British coming in as Japan's advocate and sternly advising the Yankees to horn out. They mediate a deal with Japan whereby they forego further continental expansion in Asia, and in return the Commonwealth will seek to help Japan stay more or less viable with integration into the Commonwealth preference system, including letting Japanese private firms benefit from British concessions in China. The result is to greatly annoy the American blowhard, but because the promised Splendid Little War falls through, someone else, FDR or whomever else one thinks the butterflies anoint in 1932, wins the US Presidency and reverses many of his predecessor's policies. FDR as it happened was very much pro-Navy, but also somewhat Anglophiliac. He won't be keen for close relations with Japan, but if the British want to help them out, he can at least horn out of it, as long as Japan is not attacking the Chinese any more. The USA as noted has other problems anyway! The USN is not very pleased and figures they have to stay on guard, but Japan is a far less militarist run place than OTL; the IJN gets a lot less construction. There is no war in China; presumably Japanese force has to bear down fairly hard in Korea but as noted by others, they had few problems with Taiwan. The Japanese are not violating the Naval treaty and in fact are not up to the naval strength the treaty authorizes.
Meanwhile, there is no butterfly net over things going to hell in a handbasket back in Europe, with a 1928 POD (the different American Republican blowhard guy winning the nomination versus Hoover--perhaps the POD was actually in 1924 with this other guy getting the VP nod under Coolidge) Hitler's career is much as OTL unfortunately, and he takes over in Germany setting Europe on the course to war. However when the Reich seeks to invite Japan into the Anti-Comintern Pact the British persuade the Tokyo government, which is at least nominally liberal, not to respond; Japan has no ties to the European Axis and is less fascistic. It is authoritarian, with the mandated worship of the Emperor and deep class deference and quite a bit of repression; the militarists make some noise, being among other things the closest thing to a permitted political expression for working class and peasant Japanese, but there is no pretext for war anywhere and the Army has no deeds of conquest to reinforce its claims; the Navy is not happy with lack of desired expansion but remains more moderate. Some of the OTL political assassinations take place but they are dealt with sternly by Imperial law enforcement. The Second World War breaks out as OTL, with the Axis and Soviets behaving much the same, but there is no confrontation of arms on the Soviet-Japanese border--the British are nervous and encourage the Japanese to keep a watchful eye, but it stays quiet there. The Japanese militarists get a whiff of excitement when contingency plans to strike at the Soviets, who are in fact at this point Axis co-belligerents, are considered and developed, but not executed. Japanese military forces are modernized, and Japanese industry, engaged by British war credits, manufacture weapons for the ANZAC Commonwealth forces being deployed to Europe--the Anzacs are the customers and they write the specifications, which Japanese designers don't consider optimal for their style, but can meet as long as the Entente supplies the credits and raw materials. These include pretty reasonable access at reasonable prices to Dutch East Indies oil and other "Southern resources area" goods like rubber; the militarists have dreams of seizing these zones for themselves but are again sat on by both British advocacy and the corporate "Zaibatsu" who are fairly happy arming the British. The French and Dutch place some orders too, but mainly to augment their outposts in Indochina and DEI. Then it is spring 1940, Denmark and Norway fall to Hitler, then with amazing speed the low countries and France herself. The French surrender to Hitler puts French Indochina on the wrong side of the war and now the British and Japanese war planners have contingency plans to invade Indochina, these again put on hold--the DEI administration refuses to obey the captive Dutch state under German occupation and becomes de facto part of the Commonwealth system economically, though with the Soviets and truth be told, Japan, being wild cards the DEI sits in Indonesia for the moment, not sending anything to the fight in Europe.
So it goes until Hitler strikes at the Soviets in Barbarossa--at this point all of a sudden the Soviets are Allies, and now the Japanese are urged, with yet more credits and more priority for access to resources, to build weapons and planes and the like for the Soviets. Airplanes especially, they can be flown rapidly toward the front across Siberia. In addition, the British mediate a deal with the Soviets whereby Japanese firms with technical expertise can augment Soviet prospecting and development efforts in their maritime far east, and set up factories and so forth, with the firms getting a revenue flow making worth their while along with a long term contract for access to eastern Siberian resources they help develop. The bulk of production goes to the Soviet front of course. Such deals with Western corporations were a thing the Soviets did during the 1920s under NEP, and were still doing in the early '30s for such projects as the foundation of the Urals steel works at Magnitogorsk (basing the plant design on US Steel's works at Gary, Indiana, so they are not unknown to Soviet experience. The Japanese need a lot of assurance their nationals won't be subject to Soviet justice of course!
i have a hard time figuring just how and why the USA enters WWII if at all, without war in the Pacific in the cards. It would not be too crazy to have Hitler unilaterally declare war on the USA, as he did OTL after Pearl Harbor. Certainly if the USA does come in, Japan would probably benefit from US markets being opened to her, and US credit and resources augmenting the near exhausted British to sustain Japanese contractor arms production, mainly for Soviet consumption at this point.
Postwar--the war is over when Germany collapses, no Pacific theater to settle afterward. The British House of Commons elections ought to go much as OTL, Labour winning. It would not be unlikely for someone other than Harry Truman to be US President, but I think he might still be FDR's last VP for the same reasons as OTL and so preside over US roles in the settlement. Unlike OTL there is no Pacific War and so formally speaking nothing to settle there; Japan remains as OTL in the 1920s except now with extensive investment in Soviet east Asian enterprises which the Soviets control but for a time anyway honor their wartime deal for Japanese shares in the output. The development of the Cold War might suddenly result in the capture or expulsion of those Japanese technicians and firms and cutting off the Siberian resources, but if the Soviets do that, the Americans are liable to pick up Japan, with all Korea and Taiwan in hand of course, as a Cold War client state and use Japanese industry to maintain part supplies in the Pacific for the USN and any other forces in the Pacific region far from North America. Japan joins some American brokered alliance comparable to NATO.
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another POD, or brace of them, involving the early Soviet Union:
a) the hostile one--OTL among the nations involved in supporting the "White" opposition to the Bolsheviks was Japan, again as a British client. The Japanese tried to hold some former Tsarist territory on the Pacific, OTL they were warned off by US opposition. What if British advocacy and Japanese force allows them to silence the Americans and defy the Bolsheviks, with a White Russian puppet state under Japanese protection in being north of Korea? As in the above POD this gives the Japanese a portion of Siberia to work with, now more for themselves with token attention to the interests of the regional Russians concentrated there. The Reds try to take it but are repulsed. In this scenario, the Soviets maintain a grudge and holdings north, west and farther up the coast, and gradually over the 1920s and '30s reinforce those claims, but the Japanese don't try to expand their puppet state's holdings nor do the Soviets ever move in for a kill. It almost happens in 1939-41 when the Soviets have their pact with Hitler, but Soviet caution keeps the tense peace, then as above Barbarossa throws Stalin into British, hence Japanese, arms. The Soviets affirm their acceptance of the alienation of the lost territory and to keep hands off the White puppet state, and get some patriotic aid from the exilic nation, bearing Japanese made arms; the Soviets trade resources for finished goods of the same type as above.
b) Soviet amity--the Japanese incursion into former Tsarist territory has the same resolution as OTL with Japan withdrawing. But then, in the 1920s, under NEP, the Japanese corporations are invited to partner with Soviet Far Eastern explorations, mining and industry; for some special reason or other the relationship works well, and then when the market crashes, it does not disrupt the operations in the USSR, so the Japanese companies seek more development there. The Soviets are surely being careful all this Japanese presence is not too much a security risk; with that risk in mind, they approve the higher Japanese investment and Far Eastern development is higher than OTL. Japan maintains a correct and slightly favorable relationship with Britain and both British and Soviet ties dissuade them from adventures in Manchuria and beyond. Because of the tight connection economically between Japan and USSR, relations with Britain go frosty during the period of the Berlin-Moscow Pact, which alarms the British concerned Japan might go on a pro-Axis rampage with Soviet encouragement and backing; this gives the Americans an opportunity to to say "I told you so!" and causes a general tizzy among the French and Dutch as well. However, while Japan has built warships to their agreed levels in the Washington Naval Treaty, they have not exceeded that level nor performed the sleights of hand they did OTL sneaking tonnage they denied into the ships. Unlike OTL, the Vichy France regime is not leaned on by Hitler to invite the Japanese into Indochina; then of course when Hitler does attack the USSR, the Japanese immediately are offered conciliation by Britain, USA, the Free French and DEI government in exile, as Japan goes into high gear war production, in the Home Islands, Korea, Taiwan and Soviet Far East, to feed the embattled USSR advanced materials, and being joined at the hip to the Soviets, the Imperial government goes so far as to send substantial forces of the IJA as units under overall Soviet command but acting independently on the front to defend the USSR. Similarly IJN ships are offered to the British in squadrons to incorporate into their Mediterranean and Atlantic actions, along with the mid-grade elements of the Soviet Pacific fleet--the most advanced such ships, made in Soviet yards in the East that involved Zaibatsu partners which also produced several of the Japanese Fleet, stay at home, as do the least impressive, oldest and smallest, of the Soviet eastern fleet, but the middling ships accompany about half the Japanese fleet eventually, engaged mainly in U-boat hunting. But IJA units under the Rising Sun banner are among the Red Army group that takes Berlin