The McMahon Line is the boundary drawn in March 1914 between British India and Tibet along the crest of the eastern Himalayas, running about 890 km (550 miles) from the corner of Bhutan to the Isu Razi Pass on the Myanmar border. Today its Indian section is the de facto border between India and China in Arunachal Pradesh, but China has never accepted it. Chinese maps show about 65,000 square kilometres south of the line, most of Arunachal Pradesh, as part of Tibet under the name Zangnan, or South Tibet, while India treats the line as its legal frontier. That disagreement made the line a front in the 1962 Sino-Indian War, and troops still clash on ridges along it.
Yangtse, December 2022: a line both armies patrol but neither has marked
On the night of 9 December 2022, Indian and Chinese troops fought hand to hand on a ridgeline in the Yangtse area of Tawang district, close to the Buddhist pilgrimage site of Chumi Gyatse Falls. Satellite imagery studied by independent researchers suggested that roughly 300 Chinese soldiers, reportedly moving up from a newly built border village, advanced towards two Indian posts at about 4,700 m. Both sides used nail-studded clubs and other melee weapons, and there were many injuries; six badly hurt Indian soldiers were flown to Guwahati for treatment. China’s army said Indian troops had crossed the line while its own soldiers were on routine patrol on the Chinese side, a claim the researchers did not corroborate. It was the worst clash between the two armies since the Galwan Valley fight in Ladakh in June 2020.
Yangtse is a typical McMahon Line flashpoint. The 1914 survey was inadequate and the line has never been demarcated on the ground, so in the high passes the two sides disagree about where it actually runs. Since India’s nuclear tests of 1998, both sides have generally put incidents down to disagreements of less than a kilometre over its exact location. Those differences matter because of what lies just south of them. Tawang, about 37 km from the Bum La Pass on the frontier, is home to the largest Buddhist monastery in India, and the 6th Dalai Lama, Tsangyang Gyatso, was born in 1683 at Urgelling Monastery in what is now Tawang district. That birthplace is one reason China’s claim is pressed hardest in Tawang.
Both governments have spent the years since building infrastructure towards the line. India’s Sela Tunnel, dug about 400 m beneath the Sela Pass and opened in March 2024, now gives Tawang an all-weather road link to Guwahati in Assam, and lets troops and equipment reach the border quickly even in winter. Bum La itself is a border trading point and one of the agreed meeting places where Indian and Chinese officers hold talks. It reopened to traders in 2006 for the first time in 44 years.
Simla, March 1914: how Henry McMahon drew the line
The line began as a British answer to a Chinese advance. Britain had held the Brahmaputra valley of Assam since the First Anglo-Burmese War of the 1820s, but it left the forested hill tracts to the north to the tribes who lived there, marking an administrative Inner Line in 1873 that British subjects could not cross without permission. Between 1908 and 1911 the Qing official Zhao Erfeng pushed Chinese control through the Kham region of eastern Tibet into districts next to those hills, such as Zayul and Pemako. Alarmed, the British Army General Staff proposed in 1912 a boundary along the crest of the Himalayas.
The chance came at the Simla Conference of 1913–14, where Britain, China and Tibet met to settle Tibet’s status. On 24–25 March 1914 at Delhi, Sir Henry McMahon, foreign secretary of British India, and Lonchen Shatra, the Tibetan plenipotentiary, exchanged maps and notes fixing the line. China’s delegate, Ivan Chen, was not party to those exchanges. He initialled the wider Simla Convention, but Beijing then repudiated it, and Britain and Tibet signed it as a bilateral accord. British records show Tibet accepted the new border on condition that China accepted the Convention; since China never did, Tibetans later argued the line was invalid.
The line placed Tawang, inhabited by the Monpa people, on the British side. It follows the highest ridges for most of its length and, in its western part near Bhutan, runs as a straight segment for about 30 miles. The Burmese section was later renegotiated between China and Myanmar, which is why today’s dispute concerns only the Indian part.
Even the Tibetan side’s position has shifted over time. China argues that Tibet was not sovereign and so could not sign treaties. The 14th Dalai Lama said as late as 2003 that Arunachal Pradesh was part of Tibet, but in June 2008 he explicitly recognized it as part of India under the agreement signed by Tibetan and British representatives.
From Olaf Caroe to Longju, 1935–1959
For two decades the line was almost forgotten. Simla was left out of the 1928 edition of Aitchison’s Treaties, the official collection, until Olaf Caroe, deputy foreign secretary of British India, revived it in 1935. The Survey of India published a map showing the McMahon Line as the official boundary in 1937, and a revised Aitchison volume including the Simla accord was issued with a false 1929 date to replace the recalled original. In April 1938 a small force under Captain G. S. Lightfoot reached Tawang and announced that it was Indian territory; Tibet protested, and Tawang stayed under Tibetan administration until 1951.
Independent India inherited the line in 1947 and renamed the hill tracts the North East Frontier Agency in 1954. In the same year it acknowledged Tibet as part of China. The quiet ended after the 1959 Tibetan uprising and the Dalai Lama’s flight to India. In August 1959 Chinese troops captured a new Indian outpost at Longju on the Tsari Chu, which lies a kilometre or two on the Chinese side of the line according to the mountaineer Harish Kapadia, who explored the area in 2005. On 24 October 1959, Premier Zhou Enlai proposed that both sides pull back 20 km, and he soon defined the line of actual control as “the so-called McMahon Line in the east”. China has treated the line as part of the Line of Actual Control ever since, while rejecting it as a legal border.
October 1962: the war across the line and the Sumdorong Chu standoff
In November 1961 India adopted its “Forward Policy”, setting up military posts in disputed ground, including 43 north of Zhou’s line of control. The crisis broke near the western end of the McMahon Line. On 8 September 1962 a Chinese unit attacked an Indian post at Dhola in the Namka Chu valley, just south of the Thag La Ridge and, on the map in Neville Maxwell’s history of the war, about 7 km north of the McMahon Line. On 20 October China launched a major offensive across the line.
Chinese forces advanced about 90 km from the McMahon Line to Rupa and Chaku, 65 km south-east of Tawang, and in the far east pushed 30 km to Walong. Bum La saw some of the heaviest fighting. Then China withdrew to the McMahon Line and returned Indian prisoners in 1963, leaving the line where it had been but the dispute sharper than before. The North East Frontier Agency became the union territory of Arunachal Pradesh in 1972 and a state on 20 February 1987.
The next serious crisis came in the Sumdorong Chu valley, three kilometres north of the line but south of the highest crest. Indian intelligence personnel set up an observation post there in 1984. In 1986 Chinese troops arrived first, India airlifted a task force into the area, and both armies faced each other until tensions eased in May 1987. Agreements in 1993 and 1996 created a working group on the Line of Actual Control and a set of confidence-building measures, but neither has produced an agreed map.
The Siang gorge and the Medog dam where the river crosses the line
Near the eastern end of the line, the Yarlung Tsangpo cuts through the Himalayas in one of the world’s great gorges. After flowing about 1,700 km east across southern Tibet, it enters the Yarlung Tsangpo Grand Canyon, bends around Namcha Barwa and falls through what is often described as the deepest canyon on land, about 6,009 m at its deepest. It leaves China at about 660 m above sea level, crosses the McMahon Line into Arunachal Pradesh as the Siang, and becomes the Brahmaputra on the plains of Assam.
That gorge has become the newest front in the dispute. China authorized the Medog Hydropower Station in December 2024 and began construction on 19 July 2025 in Medog County, the Pemako that Zhao Erfeng’s troops entered more than a century ago. Planned as four cascade dams with a capacity of 60,000 megawatts and expected to generate about three times as much electricity a year as the Three Gorges Dam, it is due to begin commercial operation in 2033. Because the river flows on into India and Bangladesh, the project turns a dispute about where the border runs into one about the water crossing it.
The McMahon Line is the eastern half of a longer frontier; its western counterpart in Ladakh is described in our guide to the China–India Line of Actual Control. The plateau it was drawn to fence off is covered in the geography of the Tibetan Plateau, and the river that crosses it can be followed all the way to the Bay of Bengal in the Brahmaputra, the Tsangpo of Tibet.