Why shouldn't they?
Siberia is mostly uninhabited. all the Japanese had to do was march across it until their supply lines got stretched too thin. When they reached a city they cut off the food until that city surrenderred. At this time Russia was in anarchy, so no army resisted.
S Korea was weakened signifigantly by N Korea and then Chinese attacks, so they were easy for Japan to occupy.
I really think the importance of the statement: "China would
never allow Japan to re-militarize" should
really be emphasized.
Specifically meaning if this timeline is really borne from our own timeline:
-China would do ANYTHING to block Japanese power.
-Even if South Corea defends itself successfully from a North Corean attack and later falls under Japanese control, it would be a
very tenous hold at best.
-Even if the party in power for whatever reasons withholds from intervening against Japanese action,
the
people would find it completely unacceptable and do something about it themselves. China has hundreds of thousands of inactive personnel in reserve and a good portion of them are bound to be nationalists/have lingering
As an analogy, if Greece ever encountered the possibility of Turkey becoming overly militarily powerful, they'd do
absolutely anything to kill the baby in the cradle in terms of a resurgent Turkey. The same applies here between China & Japan.
I'm basically saying the root of the problem is a strong Japan itself.
What I propose specifically, is that Siberia remains under Russian control, or changes hands into Chinese control. South Corea under Japanese control is somewhat plausible but you're going to have to either justify it with a lot of opposition being encountered everyday or a much simpler route: military-economic cooperative alliance.