The World Court resolution of Peru’s petition to change its border with Chile didn’t catch much attention beyond the Pacific coast of South America, but it matters, a lot. A century and a half ago la Guerra del Pacifico, in which Chile opposed both Bolivia and Peru, left Chile holding several hundred miles of coastal plane previously claimed by its opponents. Subsequently, in the 1929 Treaty of Lima, Peru conceded Arica to Chile, but got Tacna back. The marine border between the two countries, settled the following year, began at the coast, and extended due west at 18˚21′03′′, and there it stayed until this week.

In the meantime, conventions for setting maritime borders changed, and the current vogue amongst arbiters is to have frontiers radiate into the sea perpendicularly to the coastline. As it happens, the coast makes a sharp turn to the West just where the land borders of Chile and Peru meet. The line perpendicular to the coast veers off to the southwest, slicing off a considerable wedge of ocean, teeming with fish.