The beginning of the conflict is usually placed at the Battle of Reynogüelén, which occurred in 1536 between an expedition of Diego de Almagro and a well-organized and numerous group of Mapuche soldiers, near the confluence of the Ñuble and Itata rivers.
Campaigns of Pedro de Valdivia (1546–1553)
During the early phase of the Conquest of Chile, the Spanish conquistador Pedro de Valdivia conducted a nine-year campaign to secure the city of Santiago, which had been destroyed on September 11, 1541 by the Mapochoes under the direction of their chief, Michimalonco. Valdivia hoped to enlarge the territory under his jurisdiction and, despite injuries from a fall from his horse, resolved to take personal command of a land expedition into Araucanía.
In 1544, sent a naval expedition comprising the barks, San Pedro and Santiaguillo, under the command of Juan Bautista Pastene, to reconnoiter the southwestern coast of South America to the Strait of Magellan. The expedition set sail from Valparaíso, entered the bay of San Pedro, and made landings at what is now known as Concepción and at Valdivia, which was later named in honor of the commander. Encountering severe storms further south, he then returned to Valparaiso.
Valdivia himself set out in 1546, with sixty horsemen plus guides and porters, and crossed the Itata River and were attacked by Mapuche warriors in the Battle of Quilacura near the Bío-Bío River. Realizing that it would be impossible to proceed in such hostile territory with so limited a force, Valdivia elected to return to Santiago after finding a site for a new city at what is now Penco and that would become the first site of Concepción.
Founding of Concepción, Imperial, and Valdivia
In 1550, a new expedition was launched, consisting of a naval force under Pastene, and a land force of two hundred Spaniards mounted and foot and a number of Mapocho auxiliaries under Valdivia. They planned to reunite on the shores of the Bay of Concepción. The expedition advanced beyond the Itata River and Laja River, to the shores of the Bío-Bío River. Along the way they had several battles with groups of Mapuches as they explored the region killing many with little loss to themselves. After spending over a week in the area and encountering increasing opposition, the Spanish marched toward the Sea through the valleys of the Laja and Bío-Bío rivers, towards the coast at Penco. On the banks of the Andalién River, they camped for two days between the river and a lake, where they were attacked on the second night by a large force of Araucanians under their toqui Ainavillo in the Battle of Andalien. The night attack was defeated in a furious battle, the Spaniards suffered one killed and many wounds to men and especially their mounts. After a day treating their wounds they continued towards their rendezvous at the Bay of Concepción. There Valdivia began building a fort at what is now Penco.
On February 23, Pastene's fleet anchored in the bay, brought supplies, reinforcements and provided materials to finish the fort. On March 1 Valdivia founded here the city of Concepción del Nuevo Extremo. On March 3 of that year, the fort was completed and was attacked nine days later by the largest force of Mapuches yet seen in the Battle of Penco. This force was broken and routed despite the small size of the Spanish forces. Despite the resulting submission of the local tribes, Valdivia sent an emissary to the Viceroy of Peru, asking for additional forces; he knew that it would not be possible to complete the conquest of Araucanía with only the forces at his disposal. After reinforcement at Concepción in 1551, he organized another expedition to establish the fort La Imperial on the banks of the Imperial River. He then returned to Concepción to prepare another expedition and await the reinforcements the Viceroy had promised to send by sea.
Leaving orders that the new troops should disembark on the Tierras de Valdivia that Pastene had discovered earlier, Valdivia left with two hundred soldiers in the direction of Fort Imperial. Once he had passed it on his way south, he ordered Jerónimo de Alderete to drive inland and establish a fort, with the goal of securing his eastern flank. To this end, Alderente reached Lake Villarrica and established a fort there. Meanwhile, Valdivia's column advanced southwards and joined the reinforcements sent from Peru, under the command of Francisco de Villagra. There, the city of Santa María la Blanca de Valdivia was established. After garrisoning these new places, Valdivia returned to his base at Concepción in 1552 where rich placer gold mines were found in the Quilacoya River valley.