De Almagro promptly initiated the exploration of the new territory, starting up the valley the Aconcagua River, where he was well received by the natives. However, the intrigues of his interpreter, Felipillo, who had previously helped Pizarro in dealing with "Atahualpa", almost thwarted De Almagro's efforts. Felipillo had secretly urged the local natives to attack the Spanish, but they desisted, not understanding the dangers that they posed. De Almagro directed Gómez de Alvarado along with 100 horsemen and 100 foot to continue the exploration, which ended in the confluence of the Ñuble and Itata rivers. The Battle of Reinohuelén between the Spanish and Mapuche indigenous peoples forced the explorers to return to the north.