The fleet had nearly destroyed itself just getting there. In January 1866, during the Chincha Islands War against Spain, a combined Peruvian-Chilean squadron convoyed south from the port of Ancud toward the shipyards on Abtao, a speck of land at the head of the southern Chiloé Archipelago. The Chileans had built fortifications there, tucked at the end of a shallow, dangerous channel. The voyage was punishing. The thirty-six-gun steam frigate Amazonas, overpowered by the currents near Punta Quilque, struck a submerged rock and went down. The rest limped in and waited for reinforcements: the Peruvian corvettes Unión and América, which slipped in undetected on February 4.

Three days later the Spanish came. The frigates Villa de Madrid and Reina Blanca arrived off the inlet but hesitated at the sight of the shoals. When they finally began edging through the widest channel, allied commander Manuel Villar gave the order to fire. His ships—Apurímac, América, Unión, and the captured schooner Covadonga—had formed a line of battle to cover both inlets with their artillery. The first salvo opened at half past three in the afternoon, from fifteen hundred meters. The Spanish answered with sharper shooting, forced to fire alternately because of how the allies had positioned themselves, yet still landing blows. Apurímac took three hits at the waterline and had to pull north.

The duel settled into a grinding exchange. The América was struck six times, the Unión three—two of her crewmen killed at their stations—and the Covadonga once. The allied gunners gave back better than the casualty count suggests: the Spanish ships absorbed fourteen hits, most of them from the América and the Unión, with six men wounded. After two hours and some three thousand shots between them, the Spanish frigates faced the same wall they had started against. The allies were too well sheltered among the shoals to be dislodged. The Spaniards drew off, waited offshore through the night, and at nine the next morning gave it up entirely and returned to base.

The most lasting thing about Abtao is not who won but who was there. Aboard the allied ships served two young lieutenants whose names would later become legend across the Pacific: Arturo Prat of Chile and Miguel Grau of Peru. At Abtao they fought on the same side, comrades in a shared war against a common foe. Thirteen years later, in the War of the Pacific, they would face each other as deadly enemies at the naval Battle of Iquique—Grau's ironclad Huáscar against Prat's wooden Esmeralda. Both men became national heroes, each honored by his own country. At Abtao, before history pulled them apart, they were simply allies in a difficult fight.