In 1913–1914, representatives of the de facto independent state of Tibet and Britain met in India to define the borders of Outer Tibet.
British administrator Sir Henry McMahon drew the 550 miles (890 km) McMahon Line as the border between British India and Outer Tibet, placing Tawang and other areas within British India.
The Tibetan and British representatives devised the Simla Accord including the McMahon Line, but the Chinese representatives did not concur.
The Simla Accord denies other benefits to China while it declines to assent to the Accord.

The Chinese position was that Tibet was not independent from China and could not sign treaties, so the Accord was invalid, like the Anglo-Chinese (1906) and Anglo-Russian (1907) conventions.
British records show that the condition for the Tibetan government to accept the new border was that China must accept the Simla Convention.
As Britain was not able to get an acceptance from China, Tibetans considered the MacMahon line invalid.

In the time that China did not exercise power in Tibet, the line had no serious challenges.
In 1935, a Deputy Secretary in the Foreign Department, Olaf Caroe, "discovered" that the McMahon Line was not drawn on official maps.
The Survey of India published a map showing the McMahon Line as the official boundary in 1937.
In 1938, two decades after the Simla Conference, the British finally published the Simla Accord as a bilateral accord and the Survey of India published a detailed map showing the McMahon Line as a border of India.
In 1944, Britain established administrations in the area, from Dirang Dzong in the west to Walong in the east.
In 1913–1914, representatives of the de facto independent state of Tibet and Britain met in India to define the borders of Outer Tibet. British administrator Sir Henry McMahon drew the 550 miles (890 km) McMahon Line as the border between British India and Outer Tibet, placing Tawang and other areas within British India. The Tibetan and British representatives devised the Simla Accord including the McMahon Line, but the Chinese representatives did not concur. The Simla Accord denies other benefits to China while it declines to assent to the Accord.
The Chinese position was that Tibet was not independent from China and could not sign treaties, so the Accord was invalid, like the Anglo-Chinese (1906) and Anglo-Russian (1907) conventions. British records show that the condition for the Tibetan government to accept the new border was that China must accept the Simla Convention. As Britain was not able to get an acceptance from China, Tibetans considered the MacMahon line invalid.
In the time that China did not exercise power in Tibet, the line had no serious challenges. In 1935, a Deputy Secretary in the Foreign Department, Olaf Caroe, "discovered" that the McMahon Line was not drawn on official maps. The Survey of India published a map showing the McMahon Line as the official boundary in 1937. In 1938, two decades after the Simla Conference, the British finally published the Simla Accord as a bilateral accord and the Survey of India published a detailed map showing the McMahon Line as a border of India. In 1944, Britain established administrations in the area, from Dirang Dzong in the west to Walong in the east.
Arunachal Pradesh