Funny you mentioned Mongolia. Republic of China (Taiwan) still recognize it as part of its territory (together with mainland China).
It is the communist that ceded a lot of territory to the Soviet Union and the puppet state of Mongolia to appease the USSR by having a buffer between it and china
But that's mainly due to how the PRC operates, right?
Like when the PRC threatened invasion when the UK wanted Hong Kong to elect its own leaders, then later they can turn around and say "the British never even let the Hong Kong people vote".
Some with Taiwan / R.O.C and it's territorial claims.
