https://en.wikipedia.org/wiki/Annexation_of_Tibet_by_China
The rest of the world notes from afar that Hawaii became a US state in 1959, whilst Tibet entered the PRC way back in 1951.
In 1993, the US congress apologised for this - they didn't give it back :)
China just has a history of denying what they're doing as they're doing it.
There are so many examples online. My favourite is of a Chinese warship ramming into its own coast guard vessel as they fail to intimidate the Philippines Coast Guard.
China built a road and villages and military outposts in Bhutan, over China-Bhutan border.