After the successful capture of Valdivia Lord Cochrane sought to occupy the Chiloé Archipelago which was the last stronghold of the Spanish in Chile.

William Miller disembarked for this task but had to reembark after had been severely wounded in his leg and lost 20 men.41°49′46.48″S 73°51′18.26″W / 41.8295778°S 73.8550722°W / -41.8295778; -73.8550722 The battle was part of an effort to drive Spanish forces from the Chiloé archipelago, some of which were fortified in the San Miguel de Agüi Castle.

When Commander Saturino García arrived at the castle to reinforce the Spanish army, he found that Chilean Major William Miller had already begun combat.

[1][2] Assuming command, Captain Francisco Eréscano formed a new column, but canon and gun fire from within the castle made it difficult to advance towards it over a narrow, broken road.

The patriots found their situation made more difficult when a gunboat dispatched from San Carlos by colonial governor Antonio Quintanilla arrived and began firing on their flank.