[4] Dispatched by Peruvian president Mariano Ignacio Prado, who had rallied the South American republics in defense against Spanish aggression, the allies had sailed in convoy from the town of Ancud to the island of Abtao to await the arrival of two new corvettes acquired by Peru.

On January 16, 1866, the combined Peruvian-Chilean fleet, composed of the Peruvian frigates Apurímac and Amazonas and the recently captured and refurbished Chilean schooner Covadonga, had convoyed from the port of Ancud towards the shipyards on the little island of Abtao, at the head of the southern Chiloé Archipelago.

The allied ships (which included Apurímac, América, Unión, and Covadonga) formed a line of battle to cover both inlets of the channel with their artillery.

After two hours of battle and more of 1,500 shots from each side, the Spanish frigates, seeing that the allied fleet was well protected in her position around the shoals, decided to leave the reef and waited to go out to sea.

In his report to the Admiral Méndez Nuñez, the Spanish Captain Claudio Alvar González wrote: The most effective and precise shots came from the Peruvian corvettes Unión and América.

On March 25, the Peruvian corvettes Unión and América were sent to the Strait of Magellan to intercept the Spanish frigate Almansa, which according to intelligence reports had been dispatched from Spain to reinforce the Pacific fleet.

The rest of the allied fleet remained on the defensive in southern Chile, awaiting the arrival of the ironclads Huáscar and Independencia, which would be the factor to upset the balance of power.