The short geopolitical history is this: Tibet had an history independent of China up until the end of the first millennium. A few conflicts were waged, including a Muslim/Tibetan alliance against the Han. Unification (in a way) came under the reign of the Mongol Hordes. Both China Proper and Tibet were under the Khans' thumb. Since then Tibet has been a nominal possession with waxing and waning Chinese influence. The early 20th century saw a British invasion which evicted the last few Chinese overlords in Tibet. Tibet may have changed nominal leadership but the geopolitical ground truth stayed the same: feudalistic with large ungoverned spaces featuring roaming bands of thieves. When the People's Republic of China was founded, one of its goals was reunification with Tibet. A quick invasion forced Tibet to accept Communist rule. A failed uprising led to even greater repression.
The demographics of Tibet make it Chinese, in a way. According to the self-declared Government-in-Exile, the Communist government has been involved in population transfers making the demographic makeup more Hai Muslim and Han than Tibetan. This makes Tibet like China as well; multiethnic but ruled by Han. The recent protest in San Francisco showed how many ethnic groups have complaints against Han Communist rule.