You have (at best) a very different definition of what it would mean for China to win the war against Japan.
As far as I can infer, you apparently believe that China would win the war as long as it was not completely conquered.
That's not a definition that the vast numbers of Chinese under Japanese occupation would agree with.
For China to win the war, in my view, the Japanese would have to be driven out of all China, perhaps including Manchukuo.
China's victory would not require it to invade and conquer Japan, of course, which would something far beyond the
capabilities of China even today. China alone lacked the power to drive the Japanese out of China in 1937-45.
Could Japan have completely conquered China? I doubt it. China's just too vast of a space to occupy.
Japan might have conquered more ('Might makes right!') by practicing even more genocidal methods of warfare.
It seems unimaginable that Japan would ever become so weary of a protracted war of attrition that it would consider
withdrawing from all China in order to cut costs. No Japanese government could afford to lose that much face.
If China had fought *truly alone* against Japan, then the *best realistic outcome* for China would have been an
eventual stalemate, whereby Japan continued to occupy most of eastern China (while crushing Chinese partisans).
Please let us limit our discussion to the specifics of war.
My point is simple:
1. The reality of Empire Japan's war-machine/industry:
- They rely on materials and resources imported from the USA,
- The Japanese paid for these imports from the USA with their own wealth (gold).
- Their wealth was accumulated by providing/selling cheap goods to the US market for decades.
- The US will only accept gold-standard payments for Japanese purchases, once Japan runs out of gold, no more US imports.
- The Japanese will have to find alternative of oil, scrap steel, and other material and resources suppliers. (SEA was an inferior but viable option)
2. The reality of industrial capacity and undeveloped sources of resources around Japan:
- The Chinese mainland (central plain) back then was poor and unindustrialized, thus can provide only limited real values to the overall strength of the Japanese.
- Manchuria and Chinese mainland have the potential of industrialization, as well as untapped resources, but it will take significant time and investment to transform those to real industrial outputs of either complete products or primary-products/resources.
- Siberia is cold, vast, extremely bad transportation and infrastructure, etc.
- South East Asia are colonies of European powers, they can provide a lot of resources and materials readily for Japanese industrial consumption and the sustainment of Japanese war machine, BUT their capacity is still a lot less that that the USA, only barely sufficient to replace US imports. However, they are far from the Japanese main island industrial bases, and there are significant US naval/military presence in the Philippines that can disrupt and even destroy this maritime supply route.
3. The reality of Japanese invasion of China:
- The Japanese failed to succeed and end the war quickly, or even come to a ceasefire with the ROC.
- In the course of the long war, Japanese wealth was decrease steadily until depletion. This means they can NOT get more wealth from the invasion of China than what it cost them: it cost gold to buy resources and materials from the USA to sustain the war effort in China, but what they are getting in return from China simply can not compensate enough to balance their book at least.
Otherwise they will never run out of gold. This is also evident from point 2 previously: China mainland and Manchuria does NOT have readily available industrial outputs or sources outputs to start with. They have the potential but it takes more time and resources to transform that into production capacity.
- The Japanese failed to even form a working relationship with the occupied Chinese population, to make the only readily available resources - manpower, a meaningful addition to Japanese military industrial system. All they ever did is to fund and supply incompetent collaboration military units (皇协军), and exploited forced labors that still make no strategic differences.
- The Japanese can't effectively suppress the guerrilla warfare in occupied China. This makes it very costly and meaningless in their occupation. They loot and rob local people, gaining only their hate, yet, the measly little products/wealth a pre-industrialized society can produce is not enough to rob for the Japanese to even compensate for the industrial cost to sustain their occupation.
This is why I think the Japanese will eventually lose, regardless of whether the US help China or not.
Now if we can look at the industry/wealth part of the game in this war in Ukraine. I think it will be interesting.