In the New York Times, historian Elliot Sperling writes about the historical relationship between China and Tibet, which he says has been misrepresented by both sides of the debate:
Here are the facts. The claim that Tibet entertained only personal relations with China at the leadership level is easily rebutted. Administrative records and dynastic histories outline the governing structures of Mongol and Manchu rule. These make it clear that Tibet was subject to rules, laws and decisions made by the Yuan and Qing rulers. Tibet was not independent during these two periods. One of the Tibetan cabinet ministers summoned to Beijing at the end of the 18th century describes himself unambiguously in his memoirs as a subject of the Manchu emperor.
But although Tibet did submit to the Mongol and Manchu Empires, neither attached Tibet to China. The same documentary record that shows Tibetan subjugation to the Mongols and Manchus also shows that China’s intervening Ming Dynasty (which ruled from 1368 to 1644) had no control over Tibet. This is problematic, given China’s insistence that Chinese sovereignty was exercised in an unbroken line from the 13th century onward.