Let's say that despite the Japanese invasion of Manchuria and French Indochina the United States for some reason does not employee the oil embargo which led to the start of the Pacific War.
This is about as plausible as it gets. Even in this timeline, Imperial Japan would be seen as a major threat, and both the US and Britain would be holding an antagonistic stance vis-a-vis Imperial Japan, just looking for a reason to put the Japanese back in place.
If the Anglo-Japanese Alliance and/or Japanese-British Relations remained 'warm' and cordial during the interbellum, then possibly this antagonistic stance may be less between Britain and Japan, but not that much if the Japanese have occupied Indochina to help in their war against China.
Plus this timeline still presumes a war in Europe.
In such a scenario, my opinion is that Imperial Japan would be ultimately 'successful' in China, so long as we determine what is meant by that. Essentially without the support of the Allies, or Pacific War to distract the Japanese, the Imperial Japanese Army would have been able to maintain control of the vast majority of the East and South Chinese coastline. Even if the interior was out of their logistics capability.
The puppet government that the Japanese used while fairly unsuccessful in our timeline due to troop shortages to back it up would be a lot stronger. In some regions around Nanking, Shanghai and Northern China we might expect this puppet regime to do fairly well.
The interior would be a battlefront for years to come. Kai-shek, may not be likely to try and engage the Japanese head on due to the fact his army is fairly weak. Yet in this timeline we have to presume the British aren't coming to aid him. On the otherhand Mao will likely gain Soviet support during mid 1940s and almost certainly in the late 1940s as Stalin turns his eye East.
Aid reinforcing the Communists, will likely erode the Chinese Nationalist base to continue to prosecute a war against Japan under Kai-shek. As Mao takes centre stage the resistance movement moves to him.
With Soviet equipment the power struggle for the remains of China likely go to the Communists, at which point the Japanese success in the war in China is now of concern to the west (only if it has been allowed to go on this long).
While in our timeline the Manchurian front was almost stripped bare, in this timeline it won't be. Stalin was a cautious man, and is unlikely even with the battle success of the Red Army to prosecute a war in Manchuria to help Mao. Rather he will just back Mao.
In such a case, China becomes our timeline Korea.
The Japanese dynamic will not like Britain or the US interfering with what it will claim is its 'co-prosperity sphere'. However being pragmatic, it is likely that British and US aid will enter China on the side of 'the Allies'. At which point a form of 'eastern build up' will likely take place, as the Soviet Union makes moves to 'annex' the western remains of China as part of the Union. But leaving it under the semi-autonomous rule of Mao and other leaders.
This would guarantee that the Allies cannot 'declare war' on the rest of China without dragging the Soviet Union into conflict, Stalin would likely do this under the grounds of if he losses China, then the Soviet Union is boxed in on all fronts.
Thus the front becomes an 'armistice' that divides China.
Assuming the Cold War never goes hot, then this may likely be the state of affairs up until if/when the Soviet Union collapses, at which point there will be the 'Chinese question'.
East China, under this timeline would likely be governed by its own 'New Republic of China' government, which would be considered a Japanese puppet regime, due to IJA occupation of the country, yet this may only mirror similar events that occurred in Warsaw Pact nations during the Cold War.
The 'Imperial Japanese Empire' would be less of an Empire, and more of Japan-Korea, plus China and Manchuria as aligned states within its sphere of influence.
Korea may or may not get its 'independence' in this timeline sometime during the early Cold War. Similarly, Indochina, may likely fragment, with the Northern part joining the 'New Republic of China', while the western, southern and coastal regions Balkanise under civil/ethic tensions.
Japan will more than likely just let this happen, Indochina was never that strategic to Japan outside of Saigon (as it was then called), and even then not majorly.
The 'Japanese militarists' more than likely start to have their power base eroded sometime around the British/US involvement in China, since Japan would need a more 'civilian government' to be able to do with the change in geopolitics. The civilian nationalists would take priority over the militarist faction, although like South Korea/Britain, the state may remain heavily armed throughout the Cold War period, and even afterwards.
The Philippines were finally given their independence from America in the 1920s after a stronger referendum and are no longer a territory of the United States, but a free republic. Then let's say Japan is able to encapsulate the whole of eastern China, the free Philippines, and is able to secure the Dutch East Indies. Having obtained most of the resources it needs to form colonies the Japanese do not attack the British Empire or the United States. How much longer will Japan last as a power in the region?
^ This is total ASB without a Pacific War, whatever way you cut it. Since much of the reason to attack the DEI came from the US Embargo, and pre-war alignments of powers. Similarly, the Philippines are always going to be protected by the US against Imperial Japanese aggression, and no independent country is going to 'vote itself under' another nations auspice.
The Philippines in this timeline, might join the Allies, and so to, the DEI, thus securing the Pacific against Communism. But this would likely be ties with the US as much as Japan.
