Chinese "imperialism" is more of an example of "manifest destiny" than a sort of infinitely expansionist ideal. The idea is that each dynasty (including the present government, which I sometimes jokingly refer to as the "Mao Dynasty") will strive for suzerainty over a specific area. This is why successive dynasties will usually fill in the same borders on the map that the Qin/Han dynasties did. The invasion of Tibet is an extension of that. Although never directly ruled by China before the nineteenth century, Tibet has become a part of China's manifest destiny, and the current government will seek to encompass all the lands that it deems within that sphere of Chinese suzerainty.
In other words, in my opinion, Japan has little to do with causing the Chinese invasion of Tibet. Indeed, the Japanese "model" for invasion/occupation (if you can call it a "model") has only been halfheartedly adhered to throughout China's occupation of Tibet.
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