Chile–Peru border

The War of the Pacific

When Chile and Peru became independent, they did not touch: Bolivia’s coastal province lay between them. A dispute over taxes on a Chilean-owned nitrate company in the Bolivian Atacama led Chile to occupy the Bolivian coast in February 1879, and on 5 April 1879 it declared war on both Bolivia and Peru, which had signed a defensive alliance in 1873.^[1]

The Treaty of Ancón ended the war with Peru on 20 October 1883. Peru ceded the province of Tarapacá outright. Tacna and Arica, to its north, would be run by Chile for ten years and then a plebiscite would decide whose they were.^[1]

A plebiscite that never happened

The two sides never agreed the plebiscite’s terms, and Chile kept both provinces long past the ten years. It also took over Tarata in 1885, arguing it lay south of the Río Sama, which Peru disputed. Diplomatic relations were cut on 19 March 1910.^[1]

In 1922 the United States brought both sides to Washington and they asked President Harding to arbitrate. President Coolidge’s award of 4 March 1925 called for a plebiscite, and Chile handed Tarata back to Peru on 1 September 1925. But a commission led by General Pershing could not organise the vote, and the effort was suspended in 1926.^[1]

Tacna for Peru, Arica for Chile

After relations resumed in October 1928, President Hoover proposed a settlement, and the Treaty of Lima was signed on 3 June 1929. Article 2 gave Tacna to Peru and Arica to Chile, with a line starting at a coastal point called Concordia, 10 km north of the railway bridge over the Río Lluta, and running inland 10 km from the Arica–La Paz railway, leaving the Tacora sulphur mines in Chile. Chile also gave Peru permanent rights over the Uchusuma and Mauri water channels.^[1]

A mixed commission put up 80 pillars, from Marker No. 1 on the shore to the Bolivian tripoint, and the demarcation act was signed in Lima on 5 August 1930.^[1] In 1966 the US Geographer knew of no disputes about the line.^[1]

“The territory of Tacna and Arica shall be divided in two parts, Tacna going to Peru and Arica to Chile.”^[1]

Where does it start?

The sea was a different matter. Peru took Chile to the International Court of Justice, and on 27 January 2014 the court ruled that the sea boundary starts where the line of latitude through Marker No. 1 meets the low-water line and follows that parallel for 80 nautical miles, then turns southwest on an equidistance line.^[2]

That left a small question on land. Chile treats Marker No. 1, at 18°21′S, as the start of the border. Peru holds that the land border begins at Point Concordia, where a 10 km arc from the Lluta bridge meets the sea, and that Marker No. 1 was only placed on that arc, back from the waves.^[2] The court said the location of Point Concordia was not before it, and that the land boundary’s starting point might not coincide with the sea boundary’s.^[2]