Tibet
A brief word about Tibet (not intended as support for any particular side). Tibet was not “part of” China before the twentieth century. Understanding the relationship between Tibet and China requires understanding the Qing Dynasty, which lasted from 1644-1911.
The emperors of the Qing were from Manchuria, and they engaged in conquest southward into China and westward into central Asia. Once China was conquered, the Qing were primarily concerned with westward expansion to defeat an ethnic group of Mongols called the Zunghars. The Zunghars, like many central Asians, practiced Tibetan Buddhism, and for this reason they sought to depict themselves as defenders of the Dalai Lama. In the course of defeating the Zunghars, Qing emperors sought to take over that role. The important relationship was not between “Tibet” and “China” — it was between emperor and the Dalai Lama. There are differing accounts of the nature of that relationship, but basically the Qing was committed to prevent any rival power from gaining influence over Tibet by means of the Dalai Lama. And again, as there was little danger of anybody sending an army over the Himalayas, the primary strategic problem for the Qing had to do with Mongolia, not control over Tibet.
The Qing did not have “sovereignty” over Tibet, not in any ordinary sense of the term. The Qing did not collect taxes, nor did Qing law apply in Tibet. The Qing depicted the entire world as subject to various zones of control, and Tibet was one of those zones. We can blame Qing leaders and Tibetans for not having the foresight to see that in the future the world would be organized into nation-states, but we can’t claim that the Qing-Tibet relationship has any obvious parallel in modern international law.
The basis of China’s claim to Tibet is the same as the U.S.’s claim to New Mexico. We can be sympathetic to the sufferings of the Apache, but we’d be suspicious if Vladimir Putin began to champion their cause. And if an Apache leader took refuge in Tijuana, I can’t imagine that American public opinion would be swayed to reparations or to New Mexican autonomy. I don’t defend the way that Tibetans are being treated, but I’m also not a fan of Tibet’s champions abroad. The Chinese government acts like a colonial power largely because it itself emerged in response to colonialism. Making sense of that history seems to me crucial to figuring out what kinds of policy will promote the greater good in China and elsewhere. I hope to be writing more about it soon.