The War of the Pacific (Spanish: Guerra del Pacífico) took place in western South America from 1879 through 1883. Chile fought against Bolivia and Peru. Despite cooperation among the three nations in the Chincha Islands War, disputes soon arose over the mineral-rich Peruvian provinces of Tarapaca, Tacna, and Arica, and the Bolivian province of Antofagasta. Chilean enterprises, which largely exploited the area, saw their interests at stake when Peru nationalized all nitrate mines in Tarapaca, and Bolivia imposed a 10 cent tax on the Antofagasta Nitrate & Railway Company. The foundations of the conflict were laid in a dispute between Chile and Bolivia over part of the Atacama Desert.

The war began on February 14, 1879 when Chilean armed forces occupied the port city of Antofagasta, after a Bolivian threat to confiscate Chilean Antofagasta Nitrate Company's property. Peru attempted to mediate, but when Bolivia announced that a state of war existed, the situation deteriorated. Bolivia called on Peru to activate their mutual defense pact, whereas Chile demanded that Peru immediately declare its neutrality. On April 5, after Peru resisted both demands, Chile declared war on both nations. The following day, Peru responded by acknowledging the casus foederis.