The McMahon Line is a boundary concept and a line of demarcation that serves as the de facto border between India and China in the eastern sector of the Himalayas. It was established as part of the 1914 Simla Convention (officially the Convention Between Great Britain, China, and Tibet) to delimit the previously undefined spheres of influence between British India and Tibet. The line is named after Sir Henry McMahon, the Foreign Secretary of British India and chief British negotiator at the conference.
The line was agreed upon in maps and notes exchanged on March 24–25, 1914, and was a bilateral agreement signed by McMahon and the Tibetan plenipotentiary, Lonchen Shatra. It spans approximately 890 kilometres (550 miles) from the corner of Bhutan to the Isu Razi Pass on the Burma border, largely following the crest of the Himalayas, based on the "highest watershed principle". The Republic of China, which was present at the Simla Convention, repudiated the agreement and the line, arguing that Tibet was not a sovereign state and therefore lacked the authority to conclude treaties.
Today, the McMahon Line forms the basis for the eastern sector of the Line of Actual Control (LAC), which is the current de facto boundary between India and China. India considers the McMahon Line to be the legal international border, while the People's Republic of China disputes its legitimacy and claims the territory south of the line, which India administers as Arunachal Pradesh, as "South Tibet". This dispute over the line was a central factor in the 1962 Sino-Indian War. The line itself has not been formally amended, but its disputed status remains a core issue in the ongoing Sino-Indian border tensions.