India’s claim line in the eastern sector follows the McMahon Line. The line drawn by McMahon in 1914 Simla Treaty maps clearly starts at 27°45’40″N, a trijunction between Bhutan, China, and India, and from there, extends eastwards.
Most of the fighting in the eastern sector before the start of the war would take place immediately north of this line.
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. Thus, the Indian version of the McMahon Line moves the Bhutan-China-India trijunction north to 27°51’30″N.
Chumbi Valley on the trijunction is significant vis a vis the fact that China can reach India’s chicken neck within minutes if conflict escalates in the western sector.