After the People's Republic of China was formed on mainland China in 1949, the existing Republic of China government only retained the Island of Taiwan (sometimes called Formosa). Many would say that the ROC is the legitimate government, and the PRC is a military coup.
The next year, the PRC decided they wanted Tibet within their borders, so in 1950 they invaded the country and now occupy it. This started with the Battle of Chamdo
https://en.wikipedia.org/wiki/Battle_of_ChamdoThe independent government of Tibet, the Central Tibetan Administration, still operates in exile, and India has been kind enough to allow them operate as the Tibetan Government-in-Exile in northeast India. They advocate for the freedom of Tibet, and are a founding member of the Unrepresented Nations and Peoples Organization (UNPO)
https://en.wikipedia.org/wiki/Unrepresented_Nations_and_Peoples_Organization which represents nations and people who have no sovereignty, and aims to get them full independence and UN membership. They have been successful in several cases, such as Armenia, East Timor, Estonia, Latvia, Georgia, and Palau.
Interesting history. Worth reading about. There's a movie with Brad Pitt called Seven Years In Tibet if you'd like the Hollywood version
