king of Castile from 1158, son of Sancho III, whom he succeeded when three years old.

Before Alfonso came of age his reign was troubled by internal strife and the intervention of the kingdom of Navarre in Castilian affairs. Throughout his reign he maintained a close alliance with the kingdom of Aragon, and in 1179 he concluded the Pact of Cazorla, which settled the future line of demarcation between Castile and Aragon when the reconquest of Moorish Spain was completed. From 1172 to 1212 he was engaged in resistance to the Muslim Almohad (Almohads) invaders, who defeated him in 1195. In the same year the kings of Leon and Navarre invaded Castile, but Alfonso defeated them with the aid of King Peter II of Aragon. In 1212 Alfonso secured a great victory at Las Navas de Tolosa (Navas de Tolosa, Battle of Las) over the Almohad sultan and thereby broke Almohad power in Spain.