The 'constantly invading European armies were ....Germany...and Germany...
In the meantime, however, the Russians (USSR) invaded Eastern Europe and only left as they were removed from power by those 'Republics' when the USSR imploded.
Taking back what they had no right to take in the first place isn't any justification.
As for Taiwan (Formosa) the CCP never controlled it. That the Maoists succeeded in conquering the rest of China gives them no right to territory they never owned.
Chinese claims to the territory of others, sometimes ridiculously so, do not gain validity because the Chinese claim them, to the exclusion of other nations' claims which seem equally or more valid from a geographic perspective.
The British leased Hong Kong, they did not seize it. The lease ran out in 1999, after the British had developed Hong Kong into a major trade hub.
It was through the efforts of the Americans, primarily, that the Chinese were freed from the yoke of Japanese conquest, setting the stage for the Maoist Revolution (conquest) which has pushed the Nationalist Chinese out to Formosa.
The Russians took Manchuria from the Japanese, who conquered it as their opening gambit in WWII, setting up a puppet government in Manchuquo.
