Tibet was independent before it was taken over by force
They declared independence in 1913 after the fall of the Qing but it was not recognized by the Chinese Republican govt.
The PRC re-annexed it in 1951.
And Italy could take back large chunks of the near east.
Calais was lost in 1558 and UK monarch stopped using the title "King Of France" in 1800 so it is a settled issue.
This applies to both the rationalisation of Tibet belonging to China, and for not belonging.
If you can't force the issue (i.e Yugoslavia, Kosovo, Cyprus, Crimea) via a force of arms or diplomacy.