Think:

If you are Jiang (KMT)

what will you do? faced japan invading, USSR invading...

USSR used ccp, mongolia cp and xinjiang cp..

USSR invading tech is higher than Japan

Mao rebelling made china weaker, made USSR get a chance

Of course, Outer Mongolia is a complex.

I will say Mao 60%, KMT Jiang 20% Qing 20%

remembered KMT has 29 border talking with USSR

and asked for returning lands, included Mongolia before 1945

In 1945, it is very hard, USSR took over northeast

USSR aslo get ccp