Hmm... This is interwar Japan we are talking about. It's difficult to speak of Japanese policy as a unified whole. Certainly, there were already those in the military who wanted to turn Manchuria from purely an economic colony/sphere into a fully-fledged colony. But this was also the era of Shidehara Diplomacy, when Japan annoyed her Western peers by pressing to accept Chinese demands for tarrif autonomy.The Japanese killed Zhang Zuolin because he wanted to unify China, but I'm not sure of the details since I also read that they were pissed at him having done poorly against the central government.
I suspect that many Japanese leaders (though not in the military) would've been happy to see an end to the Warlord Era--it would be good for business. But they would not accept it if it meant giving up their economic rights in China. I believe that Zhang Zuolin was murdered for the same reason that, even during the Shidehara days, Japan deployed troops to Shandong. Japan had been willing to give up all but her economic rights there. But she was not willing to give up those. That is why they resisted when the KMT launched the Northern Expeidition, and why Zhang was killed. They felt (in my opinion) that his aligning himself with the KMT put all the Japanese factories, railroads, etc. already in Manchuria at risk.
Butterfly away Zhang Zuolin.
He was the one who shipped in all the ethnic Han Chinese immigrants from Shandong province(and other parts of Northern China.)
Without him, we may have a Korean majority in Manchuria, or even Japanese or Russian majority.
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Once Manchuria become a Chinese Majority region, I doubt there is anyway to make it a genuine "foreign" country.
That never posed much of a problem in the past, up until the early 20's 35% of Xinjiang was Han, then they were almost all expelled until after the end pf the Chinese Civil War at which point they and others returned.
The Han were the majority in Manchukou throughout its existence, but it would'nt be to difficult for either during the regime or post-war for something to happen leading to forced expulsions of large amounts leading to them only being 48-52% of the population.
I don't think the ethnic Han Chinese have to be expelled to create a lasting independent Manchuria (though it would likely make things easier). You just have to create a situation in which the people of Manchuria feel themselves to be a seperate nation and people, regardless of what we might call their ethnicity. If they experience vastly different cultural events, if they lead different lives, for a few generations, they may well develop their own identity. It happened to the English colonists in what became the US, it happened with Austria and Germany, and it happened with Taiwan and China. Given enough time, it could happen with Manchuria.