This may answer your question：
First, a correction is long overdue: the word “China” or “Chinese” cannot be found in “China’s” language or in the “Chinese” rich history records. What? Are you kidding? NO, I AM NOT KIDDING!
1)“China” and “Chinese” were imposed /used by the Europeans, a reference to the place where a bowl of porcelain was made. But, before the Europeans first laid their eyes on a china, be it a rice bowl or a tea cup, the peoples living in on that land already had a name for their country. It is called Zhong Guo. The literal translation is the “Middle Kingdom.”
2)What about its peoples? How do they address each other? There are over fifty ethnicities living in Zhong Guo. The Tibetan people are one of them. The word “Tibet” or “Tibetan” was also imposed by the Europeans. But long before the Europeans (the English) landed on the Tibet, Tibetans called themselves “bod” or “po.” This word is still in use, referring to both the place and the people.
3)So, a question like, “When did the Chinese first invade Tibet?” is an oxymoron. The fallacy is comparable to a hypothetical question: “When did Native Americans invade Navajo or Apache tribal lands? “ In America, you have Navajo, Cherokee, Choctaw, Sioux, Chippewa, Apache, etc. In Zhong Guo, they have Han, Hui, Mian, Zang, Zhuang… … about 50 plus ethnicities living on the land called the Middle Kingdom.
4)So, just like the Navajo people, who are ethnic Native Americans, the Tibetan people are an ethnic Chinese. What? How could you say that Tibetan people are an ethnic Chinese? YES, THEY ARE. But their culture, clothing, and food are so different than that of Chinese, if you object my assertion. Be calm, my friend. When you say “Chinese”, you probably refer to Han Chinese, who is only one of many ethnicities in China. But there are at least a dozen of other ethnic Chinese minorities whose culture, language, clothing, and ways of religious worship are every bit as exotic, if not more than, as those of Tibetans!
Then, what makes the Tibetan issue come to a head? A short answer is because of the English and the CIA’ s never-ending meddling. A long answer is, well, let’s explain the long one:
5)As early as the seventh century, the ethnic Tibetan and ethnic Han Chinese established close ties through royal inter-marriages; the Han Emperor’s daughter married the head of the Tibetan tribe. The alliance was cemented further into a military and political bond by a mutual agreement or a bilateral practice: Han Chinese officials (or other ethnic Chinese who took control of the dynasty) came to the Tibetan tribal court to assist in administration and defense matters, while the Tibetan court sent its officials to the Central government court. For a very long time, the Central Chinese court subsidized the Tibetan court.
6)Here is a specific in Yuan Dynasty, which was controlled by ethnic Meng Chinese, or Mongols: In the middle of the thirteenth century ( about 100 years after William the Conqueror invaded England), Tibet was formally incorporated into the Chinese territory of the Yuan Dynasty. Yuan Emperor Kublai entrusted the power of administering the Tibet region to the Sakya Sect, setting up the General Council (renamed Political Council in 1288) which was a central government organ exercising administrative power over the country's Buddhist affairs and Tibetan affairs. The Yuan government instituted the system of imperial preceptor, whose job was to confer titles on political and religious leaders; to delimit administrative divisions; to appoint local officials; to take census, to collate and stipulate revenue and taxes; to divide the Tibet region into thirteen Wan Hu (ten thousand households). The heads of Wan Hu were conferred upon and appointed directly by the Yuan Court. There were three Chief Military Commands of the Pacification Commissioners' Offices, which took charge of garrison troops and the administrative affairs of the various Wan Hu Offices in Tibet proper and other Tibetan areas. (This paragraph was from the historical records at authoritative China websites. I cannot do it otherwise since Yuan dynasty was about AD 1270—1370, and no other country can provide a detailed records on this subject. )
7)This type of integrity had kept its steadfastness until the 19th century, when China’s Qing dynasty was vitally crippled by the opium trade imposed by the English. In 1888, The English invaded Bhutan and from there launched its first attack on Tibet. The invasion met Tibetan’s fierce resistance (see http://scholar.ilib.cn/A-xzdxxb200403002.html).
In 1904 the English army, headed by Francis Younghusband, launched its second invasion on Tibet. “Younghusband slaughtered 1,300 Tibetans in Gyangzê. (see http://en.wikipedia.org/wiki/Francis_Younghusband)
In the Chinese records, the British had slaughtered over 5000 Tibetans in all at the end of the invasion. The Anglo-Tibetan Treaty of 1904 was forced upon the Tibetans. This was at a time when the Tibetans’ usual protector, China’s Emperor, could not even protect himself from the uprisings inside; neither could he keep the European powers from the outside at bay. (The Qing Dynasty ended in 1911.)
8)But In 1906 the English made the Anglo-Chinese Convention with Qing Emperor. It confirmed the Anglo-Tibetan Treaty of 1904; Britain agreed "not to annex Tibetan territory or to interfere in the administration of Tibet" while China engaged "not to permit any other foreign state to interfere with the territory or internal administration of Tibet". In the Anglo-Russian Convention of 1907, drafted by the British, Britain also recognized the "suzerainty of China over Tibet" and, in conformity with such admitted principle, engaged "not to enter into negotiations with Tibet except through the intermediary of the Chinese Government” (The above was from Wikipedia.(Seeehttp://en.wikipedia.org/wiki/Tibet#Sven_Hedin.27s_expeditions)
9)In 1914, China was in chaos. The English seized the chance to shovel “the Simla Convention” down Tibetan and China’s throats. By this treaty the English would partition Tibet into two, Inner Tibet and Outer Tibet. (By the way, this treaty was the main cause of border dispute between India and China.) China resisted it, to no avail. In the end, Tibetans signed the treaty under the English pressure, whereas the China government refused to sign. Soon the World War I started, absorbing the whole world into it. When it ended, China found itself emerged as a victim at the hand of the Japanese despite that China sided with the Allies. So the revolution ensued. Young Chinese were going abroad in droves to find ways to save the country. They got support from Russia while the English and Americans refused (for the obvious reason that they intended to keep their Extraterritoriality, a form of colony over which China had no sovereignty). This was a main reason that the Chinese Communist Party was growing fast and popular among the Chinese mass. Then, the War of Anti-Japanese invasion, the World War II and the Civil War ensued. Tibet was in neglect.
10) In 1949, the Peoples’ Republic of China was established. Mao Zedong declared that the new China shall “abolish all unequal treaties forced upon the peoples of China by the foreign powers.” This certainly includes treaties coerced upon the ethnic Tibetans by the English. The Communist Party Army, called Peoples Liberation Army (PLA), chased the Nationalist Army, supported by the US and UK, down to the far ends of China territory, including the Tibet. Tibetans joined forces of the Nationalist army to resist. They were simply not a match. But Mao Zedong had no intention to crush or overthrow Dalai Lama regime. It was kept intact. Dalai Lama’s theocracy and its privilege basically were same as it had been. Dalai Lama was all happy when he was invited to Beijing to sign a agreement with Mao in 1951. Four years later he and another Lama were at top of the Central Chinese government and the PLA undertook the defense of Tibet. The century-old tradition came back; and Dalai gave a praise speech about it when he addressed the National Peoples Congress. All went well until,
11)Well, here comes a part that is hard to relate. Even if I can describe it, you would probably not believe it. Part of reason is that the Hollywood, the media and the publishers had rarely exposed it ( for the purpose we do not yet know); Part of reason is its uniqueness in the way that Dalai Lama regime governs its people before 1959. It was a serfdom in an extreme form plus a theocracy in highest degree. Here is a story told by my friend, who happened to be an aid to another Lama, Banchan, Banchan is the second in Tibetan theocratic hierarchy. But unike Dalai Lama, Banchan stayed in China and he lived in Beijing. About 1986, Banchan took a tour in the southwest Tibet, He took my friend with him. What shocked my friend (he is not an ethnic Tibetan but an ethnic Man Chinese) is that the region was so religious, so lama worshiping, that the whole entourage could hardly move Banchan out of crowd for the next village, What blocked their progress is not only the pious crowd, but also the money that thrown at them by the Tibetan villagers. For every 10 -20 meters, Banchan’s limo driver and my friend had to stop in order to remove the paper money piling up over the windshields. This was about 1985, when China was about to develop, the interior China remained poverty-stricken, even so inside the Tibet. But those pious villagers gave Banchan all they got, When money was gone, they start throwing bronze bracelet, bead necklace or whatever they believe value-worth at the entourage. This was their first time to see a Banchan, a reincarnation in their belief. They had no reason to reserve, since their life was m