After toqui Ainavillo's defeat in the Battle of Andalien, he gathered tens of thousands of warriors from the Arauco and Tucapel regions to reinforce his depleted 15,000 man army for an attack on Valdivia's new settlement at Penco.

Inside the fort there was some discontent among the conquistadors at being so hemmed in and letting the Mapuche gain courage thinking the Spaniards were afraid of them by not fighting in the field where their cavalry had always been able to defeat these enemies.

Meanwhile, Ainavillo's command that had been previously defeated at Andalien, was recognized by the Spaniards, and they also saw that the Mapuche divisions were separated from each other in a way that prevented them from aiding one another easily.

Valdivia realized that Alderete had forced his hand and sent out Pedro de Villagra with the rest of the cavalry and directed the softening up of Ainavillo's command by volleys of their firearms and artillery.

[8] The battlefield was littered with discarded weapons, 300 dead Indians killed in the clash with Ainavillo alone, according to Vivar, 4,000 was the total Mapuche loss after the pursuit, according to Lobera, and 200 were captured including many leaders of the army.