The Chincha Islands War emerged from a backdrop of 19th-century Latin American struggles for sovereignty against imperial powers, particularly Spain. In the mid-1860s, Spain sought to reassert its influence over its former colonies, especially after several countries had successfully declared their independence. The Chincha Islands, which belonged to Peru, were crucial due to their abundance of guano, a lucrative fertilizer that attracted foreign interest. Tensions exacerbated when Spanish naval forces began to seize Peruvian vessels and occupied the islands in late 1865.

In response to Spanish aggression, Peru and Chile, motivated by common interests and threats from Spanish forces, created a defensive alliance. The agreement meant they would collaborate militarily, a strategic move underscoring the importance of regional solidarity against colonialism. Armed conflict erupted as both nations prepared their forces to confront the Spanish threat. Notably, Chile contributed naval power, while Peru mobilized land troops to defend the Chincha Islands. This combined effort showcased the evolving dynamics in South America as nations united to preserve their independence and resources against imperial ambitions.

The military engagement saw several skirmishes and strategic maneuvers, marking a turning point in Chile-Peru relations, with increased collaboration addressing mutual security concerns. The war also fostered a sense of nationalism among South American nations, emphasizing the struggle against colonial dominion. By pooling their military resources and strategies, Peru and Chile incrementally pushed back Spanish advancements, leading to the eventual recognition of their autonomy and territorial claims.