The idea that Tibet became part of China in the 13th century is a very recent construction. In the early part of the 20th century, Chinese writers generally dated the annexation of Tibet to the 18th century. They described Tibet’s status under the Qing with a term that designates a “feudal dependency,” not an integral part of a country. And that’s because Tibet was ruled as such, within the empires of the Mongols and the Manchus. When the Qing dynasty collapsed in 1911, Tibet became independent once more.
From 1912 until the founding of the People’s Republic of China in 1949, no Chinese government exercised control over what is today China’s Tibet Autonomous Region. The Dalai Lama’s government alone ruled the land until 1951.
Marxist China adopted the linguistic sleight of hand that asserts it has always been a unitary multinational country, not the hub of empires. There is now firm insistence that “Han,” actually one of several ethnonyms for “Chinese,” refers to only one of the Chinese nationalities. This was a conscious decision of those who constructed 20th-century Chinese identity. (It stands in contrast to the Russian decision to use a political term, “Soviet,” for the peoples of the Union of Soviet Socialist Republics.)
There is something less to the arguments of both sides, but the argument on the Chinese side is weaker. Tibet was not “Chinese” until Mao Zedong’s armies marched in and made it so.
Don’t Know Much About Tibetan History, by Elliot Sperling, director of the Tibetan Studies program at Indiana University’s department of Central Eurasia Studies. http://www.nytimes.com/2008/04/13/opinion/13sperling.html?ref=opinion