no country has the right to occupy territory that isn't somehow rightfully theirs and even then it's difficult to justify any kind of occupation.
Really? Then you must leave the Netherlands at once, because your country and every other country in history was obtained by some sort of occupation. Except for maybe some tribes in South America.
Last time I've checked we aren't occupying anyone's territory but our own, but in a way you are right. It's difficult, if not impossible in most cases to determine whether someone has the 'right' to live somewhere. Still, all kinds of literal occupation is wrong in my opinion and most cases of occupation are actually pretty obvious. Take Israel for example and the Syrian Golan heights.
It doesnt matter who is right or more right, the Chinese will occupy whatever country they want if there is no one around to stop them, and noone strong enough cares that much about Tibet because it doesn't offer any resources. Thats the harsh reality.
Just like with Taiwan China is convinced that they rightfully own these countries.
By the way Tibet
does have vast deposits of metal resources which are of great interest to the Chinese. It's the number one reason why they build a railroad going to Tibet.
http://money.cnn.com/2007/02/20/magazines/fortune/lustgarten_china.fortune/index.htm