Mountain borders are international boundaries that follow a mountain range, usually along its crest or its watershed, the line that divides rivers flowing to one side from rivers flowing to the other. They include some of the longest frontiers on Earth: the Argentina–Chile border runs 5,308 kilometres down the Andes, the China–Nepal border crosses 1,389 kilometres of the Himalayas over the summit of Everest, and the Alps and Pyrenees carry the Italy–Switzerland and France–Spain lines. Mountains look like the most permanent boundary a state could want, yet they produce some of the hardest disputes to settle, because a ridge can be read in more than one way and the ice that once fixed a line is now melting.
Galwan Valley, June 2020: the ridge nobody had marked
The most serious mountain-border crisis of recent years happened in the Galwan Valley in Ladakh, where a fast-flowing river rises on the eastern side of the Karakoram and runs west into the Shyok, part of the Indus system. The valley lies on the Line of Actual Control, the notional line separating Indian-controlled from Chinese-controlled territory. It is not a boundary either side has accepted as final: India claims all of Aksai Chin, which China administers, and China claims Arunachal Pradesh, which India administers.
In late May 2020 Chinese forces objected to Indian road building in the valley. On the night of 15–16 June, troops fought hand to hand on a steep ridge above the river. India reported that 20 of its soldiers died, several of them after falling from the ridge or into the icy water, others from injuries and hypothermia. China's defence ministry confirmed casualties but at first refused to give a number; in February 2021 its Central Military Commission posthumously honoured four soldiers killed in the clash. On 7 September 2020 shots were fired along the line for the first time in 45 years, each side blaming the other.
The clash showed why high-altitude borders are so fragile. There is no publicly available map depicting the Indian version of the Line of Actual Control, and in 1993 the two governments agreed to respect the Line of Actual Control without ever demarcating it on the ground. Winter temperatures in parts of Ladakh and Aksai Chin fall to about −40 °C, so both armies build roads, bases and heliports to hold positions year-round. Troops pulled back in stages, from Pangong Lake in February 2021 and Gogra in August 2021, and in December 2024 India's foreign minister told parliament that disengagement in eastern Ladakh had been achieved in full. Further east, the boundary follows the McMahon Line, which Britain and Tibet agreed at the 1914 Simla Convention along roughly 890 kilometres of the Himalayan crest; China has never accepted it, and it was one of the fronts of the October–November 1962 war. Our guide to the McMahon Line covers that sector in detail.
Crest, watershed or highest peaks: the Andes test of 1881–1902
Every mountain boundary depends on a drafting choice, and the Andes show what happens when the wording and the landscape disagree. The Boundary Treaty signed in Buenos Aires on 23 July 1881 set the Argentina–Chile line, as far south as the 52nd parallel, along "the highest summits" of the Andes "which divide the waters". In the north that was easy, because the highest peaks and the continental divide coincide.
The same drafting device runs through the Himalayas wherever the two sides have actually agreed a line. China and Nepal confirmed their frontier in the 1960 Treaty of Peace and Friendship and a 1961 boundary treaty, and Article 1 of that treaty starts the border at the point where the watershed between the Kali and Tinkar rivers meets the watershed of the Karnali's tributaries, at the head of the Tinkar Valley in Nepal's Darchula District. From there the 1,389-kilometre line runs south-east along the main range and across the summit of Mount Everest. Written in watersheds rather than coordinates, it is the method at its simplest: named rivers, a surveyed divide and two governments that both signed.
In Patagonia they do not. Glaciers have carved valleys through the range, so many lakes on the eastern side drain west to the Pacific. Argentina read the treaty as a line along the highest peaks, which favoured it; Chile read it as the watershed, which favoured Chile. As tension rose, the two governments agreed in 1896 to seek outside arbitration and in 1898 formally asked Britain, and in 1902 an award in the name of King Edward VII drew a compromise that followed neither principle strictly. It left several disputed lakes split by the border, which is why O'Higgins Lake in Chile is San Martín Lake in Argentina, and why a number of Patagonian lakes carry a different name on each shore.
The settlement is commemorated on the old road between Mendoza and Santiago. At the La Cumbre pass, 3,832 metres up, the statue of Christ the Redeemer of the Andes was unveiled on 13 March 1904 to mark the peaceful resolution. The road over the pass opens only in summer, and most traffic now uses the Cristo Redentor tunnel beneath it. For the range itself, see our guide to the Andes mountains.
Laguna del Desierto and the unfinished Southern Patagonian Ice Field
The 1902 award did not end the argument. Near the lake that Chile calls O'Higgins and Argentina calls San Martín, the arbitrators set boundary marker 62 and drew the line to Mount Fitz Roy, but the ground between was barely surveyed. On 6 November 1965 a patrol of Chilean Carabineros met a much larger unit of the Argentine National Gendarmerie in the Laguna del Desierto area; Lieutenant Hernán Merino was killed and a sergeant wounded, setting off a diplomatic crisis. The two countries eventually took the question to a new tribunal, which on 21 October 1994 awarded the 481-square-kilometre disputed area to Argentina, and rejected Chile's request for reconsideration on 13 October 1995.
Further south the line crosses the Southern Patagonian Ice Field, about 16,480 square kilometres of ice stretching roughly 350 kilometres between latitudes 48°15′ and 51°30′ south. It feeds glaciers on both sides, including Perito Moreno and Upsala in Argentina and Pío XI in Chile. An agreement of 16 December 1998 redefined the border between Mount Fitz Roy and Mount Daudet, but the two governments could not agree on the stretch between Fitz Roy and Mount Murallón, which remains undefined. Chilean maps show the 1998 line; Argentine maps still show Argentina's earlier claim. More than a century after the treaty that tried to settle it, part of the world's third-longest land border still has no agreed position.
Melting Alpine ice, the Nathu La pass and Llívia
In the Alps the problem is not interpretation but climate. The Italy–Switzerland border runs 744 kilometres from Mont Dolent to Piz Lad, crossing the Matterhorn, whose summit stands at about 4,478 metres, and reaching 4,618 metres at the Grenzgipfel just east of the Dufourspitze. Much of it was defined along glacier ridges and watersheds, and as the ice retreats, the watershed itself moves. In May 2023 a joint Italian-Swiss commission agreed to redraw a section that crosses an Alpine peak where melting glaciers had shifted the historically defined line; Switzerland approved the treaty in September 2024, while Italy had still to sign it. The Matterhorn, often claimed to be the most photographed mountain in the world, sits on the main watershed that the line follows.
Passes are where mountain borders become usable, and where they are most tightly controlled. Nathu La, at 4,310 metres in the Dongkya Range, links Sikkim in India with Yadong County in Tibet. J. W. Edgar surveyed it in 1873 and described Tibetans using it for trade, and Francis Younghusband crossed it in 1903–04. India and China sealed it after the 1962 war and fought there in 1967, and it reopened for limited trade in 2006, restricted to certain goods and certain days of the week.
The Pyrenees show how an old mountain treaty can leave odd remnants. The Treaty of the Pyrenees, signed on 7 November 1659 on Pheasant Island in the Bidasoa River, gave France Roussillon and the northern half of Cerdanya and fixed the new frontier roughly along the range's drainage divide. It transferred only the "villages" north of the mountains, and Llívia, the old capital of Cerdanya, counted as a town. It therefore stayed Spanish, and today it is an exclave of 1,562 people (2025) surrounded by France, separated from the rest of Spain by a strip of French territory about 1.6 kilometres wide that a shared road crosses. The border was not fully settled until the Treaty of Bayonne of 1856, and Pheasant Island is still a French-Spanish condominium.
Taken together, these cases show that a mountain range does not settle a border on its own. The Andes needed a British arbitration and a 1994 tribunal, the Himalayas still have a line of control that was never marked, and the Alps are being redrawn because the ice is shrinking. The 685-kilometre Franco-Spanish frontier, by contrast, has held since 1659, with the Treaty of Bayonne of 1856 settling its details. Where both sides accept the wording and the landscape stays put, a ridgeline makes a stable border; where either changes, the peaks turn into places to argue over, as our guide to the Pyrenees mountains and their frontier villages shows.