The Boundary Treaty of 1881 between Chile and Argentina is signed in Buenos Aires.

The Boundary Treaty of 1881 between Chile and Argentina was a pivotal agreement that helped to resolve longstanding territorial disputes in South America, particularly in the Patagonia and Andes regions. Signed on July 23, 1881, in Buenos Aires, the treaty aimed to bring clarity and stability to borders that were previously vague or overlapping due to differing colonial-era interpretations. The treaty divided the land along the Andes Mountains, using the highest peaks that divide watersheds as the principal boundary line—an approach influenced by the uti possidetis juris principle, which maintains pre-independence administrative borders.

One of the most significant outcomes of the treaty was the division of the southern territories. Chile gained sovereignty over the entire Strait of Magellan, a strategically vital passage, while Argentina secured most of Patagonia to the east of the Andes. The treaty also stipulated that both countries would respect the sovereignty of Tierra del Fuego, dividing it longitudinally, with Argentina receiving the eastern part and Chile the western portion. This compromise helped avoid potential conflict, but it left certain zones—especially in the southern Andes—ambiguous, sowing the seeds for future disputes.

Though it brought temporary peace and laid the groundwork for bilateral cooperation, the Boundary Treaty of 1881 wasn’t the final word on territorial matters. As explorations advanced and the value of the disputed territories increased, especially due to natural resources and navigation rights, tensions reemerged in the 20th century. Several boundary arbitration processes and treaties followed, including the 1902 arbitration mediated by the United Kingdom. Still, the 1881 treaty remains foundational in understanding the modern borders between Chile and Argentina, symbolizing an early and essential attempt at peaceful resolution in a region marked by post-colonial power struggles.