haha... but seriously, the Chinese justification for the occupation of Tibet is that one of the Mongol Empires that absorbed China included Tibet.
If the Mongol empires serve as the Chinese definition of what China's natural borders are, most of Asia, including Russia are all provinces in open rebellion.
On the other hand, China doesn't have any real claim to Taiwan, except that the dominant ethnic group there are Chinese. So what we have here are two logics for Chinese conquest. Historical borders of Empires of which China was a part of, and an inherent right to rule over all ethnic Chinese, regardless of historical borders.
Mongol empire war-time borders in 1241 strechted to Legnica (Western Poland). Ooooff, luckily I live about 60 kilometers West from there. ;)