British India and China gained a common border in 1826, with British annexation of Assam in the Treaty of Yandabo at the conclusion of the First Anglo-Burmese War (1824–1826). Subsequent annexations in further Anglo-Burmese Wars expanded China's borders with British India eastwards, to include the border with what is now Myanmar.
In 1913–14, representatives of Britain, China, and Tibet attended a conference in Simla, India and drew up an agreement concerning Tibet's status and borders. The McMahon Line, a proposed boundary between Tibet and India for the eastern sector, was drawn by British negotiator Henry McMahon on a map attached to the agreement. All three representatives initiated the agreement, but Beijing soon objected to the proposed Sino-Tibet boundary and repudiated the agreement, refusing to sign the final, more detailed map. After approving a note which stated that China could not enjoy rights under the agreement unless she ratified it, the British and Tibetan negotiators signed the Simla Convention and more detailed map as a bilateral accord. Neville Maxwell states that McMahon had been instructed not to sign bilaterally with Tibetans if China refused, but he did so without the Chinese representative present and then kept the declaration secret.[7]
V.K. Singh argues that the basis of these boundaries, accepted by British India and Tibet, were that the historical boundaries of India were the Himalayas and the areas south of the Himalayas were traditionally Indian and associated with India.[25] The high watershed of the Himalayas was proposed as the border between India and its northern neighbours.[25] India's government held the view that the Himalayas were the ancient boundaries of the Indian subcontinent and thus should be the modern boundaries of British India and later the Republic of India.[25]
Chinese boundary markers, including one set up by the newly created Chinese Republic, stood near Walong until January 1914, when T. O'Callaghan, an assistant administrator of North East Frontier Agency (NEFA)'s eastern sector, relocated them north to locations closer to the McMahon Line (albeit still South of the Line).[1] He then went to Rima, met with Tibetan officials, and saw no Chinese influence in the area.[1]
By signing the Simla Agreement with Tibet, the British had violated the Anglo-Russian Convention of 1907, in which both parties were not to negotiate with Tibet, "except through the intermediary of the Chinese Government", as well as the Anglo-Chinese Convention of 1906, which bound the British government "not to annex Tibetan territory."[26] Because of doubts concerning the legal status of the accord, the British did not put the McMahon Line on their maps until 1937, nor did they publish the Simla Convention in the treaty record until 1938. Rejecting Tibet's 1913 declaration of independence, China argued that the Simla Convention and McMahon Line were illegal and that Tibetan government was merely a local government without treaty-making powers. In 1947, Tibet requested that India recognise Tibetan authority in the trading town of Tawang, south of the McMahon Line. Tibet did not object to any other portion of the McMahon line. In reply, the Indians asked Tibet to continue the relationship on the basis of the previous British Government.[7]
Tibetan officials continued to administer Tawang and refused to concede territory during negotiations in 1938.[1] The governor of Assam asserted that Tawang was "undoubtedly British" but noted that it was "controlled by Tibet, and none of its inhabitants have any idea that they are not Tibetan."[1] During World War II, with India's east threatened by Japanese troops and with the threat of Chinese expansionism, British troops secured Tawang for extra defence.[1]
China's claim on areas south of the McMahon Line, encompassed in the NEFA, were based on the traditional boundaries.[25] India believes that the boundaries China proposed in Ladakh and Arunachal Pradesh have no written basis and no documentation of acceptance by anyone apart from China.[25] Indians argue that China claims the territory on the basis that it was under Chinese imperial control in the past,[25] while Chinese argue that India claims the territory on the basis that it was under British imperial control in the past.[27] The last Qing emperor's 1912 edict of abdication authorised its succeeding republican government to form a union of "five peoples, namely, Manchus, Han Chinese, Mongols, Muslims, and Tibetans together with their territory in its integrity"[28] However, V.K. Singh cites the presence of the Mauryan Empire and Chola Dynasty in regions India does not place a claim to but which were heavily influenced by Indian culture.[25]
India's claim line in the eastern sector follows the McMahon Line. The line drawn by McMahon on the detailed 24–25 March 1914 Simla Treaty maps clearly starts at 27°45’40"N, a trijunction between Bhutan, China, and India, and from there, extends eastwards.[7] Most of the fighting in the eastern sector before the start of the war would take place immediately north of this line.[1][29] However, India claimed that the intent of the treaty was to follow the main watershed ridge divide of the Himalayas based on memos from McMahon and the fact that over 90% of the McMahon Line does in fact follow the main watershed ridge divide of the Himalayas. They claimed that territory south of the high ridges here near Bhutan (as elsewhere along most of the McMahon Line) should be Indian territory and north of the high ridges should be Chinese territory. In the Indian claim, the two armies would be separated from each other by the highest mountains in the world.
During and after the 1950s, when India began patrolling this area and mapping in greater detail, they confirmed what the 1914 Simla agreement map depicted: six river crossings that interrupted the main Himalayan watershed ridge. At the westernmost location near Bhutan north of Tawang, they modified their maps to extend their claim line northwards to include features such as Thag La ridge, Longju, and Khinzemane as Indian territory.[7] Thus, the Indian version of the McMahon Line moves the Bhutan-China-India trijunction north to 27°51’30"N.[7] India would claim that the treaty map ran along features such as Thag La ridge, though the actual treaty map itself is topographically vague (as the treaty was not accompanied with demarcation) in places, shows a straight line (not a watershed ridge) near Bhutan and near Thag La, and the treaty includes no verbal description of geographic features nor description of the highest ridges.[7][30]