Tibetans are very different from Han Chinese in terms of linguistic, religion as well as cultural terms. Just like the Xinjiang region of China where Ugiur community are more like Kyrg community, preaching Islam. Tibetans were isolated until Dzungar tribesman launched an invasion against Tibet in 1717. That was when the Chinese sent their soldiers to save Tibet. Once invasion prevented ( by Chinese), Chinese emperor Qianlong decided to spread its influence over Tibet. Qianlong was the fifth emperor of Qing dynasty. The military was stationed at Lhasa and commissioners known as Ambans were posted. Chinese never declared that Tibet belongs to them at that time but also did not said that Tibet is fully autonomous. Tibetans accepted Ambans, not having other major choices. Things went just like that until, in 1895, 13th Dalai Lama Thubten gained maturity. Dalai Lama acts as a spiritual head and also as political head of Tibet. Earlier Dalai Lamas failed to gain maturity and used to die earlier mysteriously. Thubten and Tibetans were feeling a negative impact of Chinese in their land, as Ambans were corrupted and irresponsible. Also in 1893, Chinese and Britishers signed an agreement allowing duty-free trade between China and India(via Sikkim). This agreement ignited Tibetans and they were not involved in the agreement, even when trading was via Tibet (In fact today also, about all India-Sino border is actually India-Tibet border) This move clearly said that Britishers were reluctant to recognize Tibet as a state. Thubten decided to take this issue to court to Tsar Nicolas II, in order to seek support and strong back from Russia.
However, in 1899, the Tory government selected new Viceroy for India. Newly selected Viceroy Curzon was highly hostile to Russia. He wrote multiple letters to Dalai Lama Thubten to properly follow 1893 agreement and not to seek support from Russia. However, these letters returned back from Lasha to Viceroy without even being opened. This made Curzon furious. He ordered Young Husband for a mission in Lasha (Expedition team of Husband in featured image). By the time Britishers reached Lasha, Thubten already fled the city. Britishers signed treaties with other Tibetans officials in 1904 which gave Britishers power to control trading in Tibet. China further made a convention of 1906 with Britishers where they undermined Tibetan autonomy by not allowing Tibet to sign any agreement or treaty with Third party state. Also refusing Britishers to assert any influence of theirs in Tibet and not to annex it. Convention of 1907, signed between Russia and China, made clear for Russia, not to interfere in the issue of Tibet.
Despite all the conventions, Britishers were feeling that China can even try to expand its influence over the North-East region. The Tory government also felt possible involvement of Russia in the region, which was not at all desirable. With all the tussle, Qing dynasty collapsed in 1912 and Sun-Yat-Sen established a republic in China. With the collapse of Qing dynasty in Peking, Tibet felt a golden opportunity to rebel and gain independence. Thubten returned back to Lasha in 1912 and with other Tibetans, revolted against China and pushed ambans out of the area. Britishers also felt that maybe independence of Tibet and help Britishers, by creating an intermediate state between India and China, thus reducing tensions. Hence Sir John Jordan wrote a diplomatic letter to Peking, declaring complete autonomy of Tibet from then onwards. In order to establish good relations with Lasha, Britishers made a point clear that Tibet would be as independent as Britain or China. It was a violation of the 1904 convention, but Britishers said that the 1904 convention allowed China to have its influence in China but not to have complete administration of Tibet, which China was doing. China spoke and protested against such move by Britisher, but with no result.
Things considerably changed when Sir Arthur Henry McMahon was appointed as foreign secretary. He started conducting conferences with Chinese and Tibetan ambassadors. He suggested that Tibetans can claim Inner Tibet while China can act as an admin over outer Tibet. Tibet favored this while the Chinese ambassador was anti. This led to bilateral talks between British and Tibet, which China also declared not acceptable. Finally, McMahon formed the McMahon line. He defined Tibetan territory with a red line and also claimed 50,000 square of territory inside the British territory (the region in present Arunachal Pradesh). Tibet did not object, while China refused to accept the McMahon line. McMahon line was more stressed about Southern Tibet border, however, the line was also made in Northern region. Region of Aksai Chin was mentioned for India. But British Raj never stationed its forces in Aksai Chin, as McMahon line was never taken seriously for the northern borders.
When India gained Independence, political leaders took McMahon line for granted as territory of India, hence claiming the region of Aksai Chin. This was the main and major reason for Indo-Sino war of ’62. Similarly, as China refuses to accept McMahon line, it recognizes Arunachal Pradesh of India as “South Tibet”, whereas Arunachal Pradesh is internationally recognized to be under India geographically as well as politically.