@simon33:
The strange part about the Mongolian rule is if USSR attacks Japanese held Suiyuyan, Chahar etc it nullifies the Mongolian rule.
But yes, in your example Mongolia goes USSR providing that USSR hasn’t nullified the rule yet.
That’s one of the few “realism” in G40 where Mongolia didn’t care for China as it had closer ties with USSR hence why they join USSR if they get invaded but not if China falls. I think at some point, some Chinese warlords encroached Mongolia and thus set off some resentment between the two countries and Mongolia could not care less about the civil war and Japan showing up made it worse as the Japanese could care less what about was Mongolian.