Argentina–Chile border

The highest peaks that divide the waters

Argentina and Chile first agreed a boundary on 30 August 1855, keeping the colonial status quo of 1810. A treaty of 1881 ran the line along the main range of the Andes as far south as latitude 52° S.^[1]

A protocol of 1 May 1893 said Argentina would hold everything east of “the line of the highest peaks which divide the waters” and Chile everything west. That assumed the crest and the water divide were the same line, but in high mountains they often are not.^[1]

“hold in perpetuity all territory to the east of the line of the highest peaks which divide the waters while Chile was to hold in perpetuity all territory to the west of that line.”^[1]

Three British arbitrations

A treaty of 17 April 1896 provided for arbitration by the British Crown if the two sides disagreed south of 26°52′45″ S. They did, in four stretches where the highest peaks and the water divide parted company.^[1] King Edward VII’s award of 20 November 1902 gave Argentina the upper basins of the disputed rivers and Chile the lower basins, and the line was marked with pillars that were often far apart.^[1]

In the Palena–California valley, pillars 16 and 17 were nearly 24 miles apart, settlers from both countries moved in, and disputes followed. Chile asked Britain to arbitrate again in 1964, and a tribunal chaired by Lord McNair split the area in 1966, about 30% to Chile and the rest to Argentina.^[1]

Three islands

At the far south the border follows the eastern part of the Beagle Channel, named after the ship in which Charles Darwin explored the area in 1833–34.^[2] Picton, Nueva and Lennox, three islands at the channel’s eastern end, were disputed from the 1840s, and the dispute almost led to war in 1978. It ended when a treaty giving the islands to Chile came into force on 2 May 1985.^[2]

The ice field

The last open question is in the Southern Patagonian Ice Field, the third-largest ice cap after Antarctica and Greenland, where ice close to 1,000 m thick hides the ground beneath.^[3] Chilean and Argentine maps of it began to diverge in the 1950s. The two countries signed an agreement in 1998, but in 2016 the ice field was still described as the last boundary issue pending between them, with maps on each side showing it differently.^[3]