TECHNICALLY... Tibet had never been a member of the LoN or the UN, and its independence was not recognized by another sovereign nation. Therefore, regarding international law, Tibet is not occupied or was annexed because it was never officially independent to begin with. If you look at it in a historical perspective, Tibet had been a tributary of Qing China till the early 20s, after which it fought against chinese warlords, the kumotang, and later came under the occupation of the CCP.
This is, of course, the opinion of this humble BOI, which is not that of an expert...