On the political level, some powerful Manchu emperors succeeded in exerting a degree of influence over Tibet but they did not incorporate Tibet into their empire, much less China. It was entirely ineffective by the time the British briefly invaded Tibet in 1904.
From 1911 to 1950, Tibet successfully avoided undue foreign influence and behaved, in every respect, as a fully independent state.
Michael van Walt is an international legal scholar and a visiting professor at the Institute of Advanced Study.
In the article below, he explains Tibet's legal status.
Tibet has maintained a unique culture, written and spoken language, religion and political system for centuries.
Tibet remained neutral during the Second World War, despite strong pressure from China and its allies, Britain and the USA.To claim that Tibet became a part of China because both countries were independently subjected to varying degrees of Mongol control, as the People’s Republic of China does, is absurd.This relatively brief period of foreign domination over Tibet occurred 700 years ago.The Ming Dynasty, which ruled China from 1368 to 1644, had few ties to and no authority over Tibet.On the other hand, the Manchus, who conquered China and established the Qing Dynasty in the 17th century, embraced Tibetan Buddhism as the Mongols had and developed close ties with the Tibetans.