Alfonso was born as the son of King Ordoño I of Asturias. From birth, he was destined to continue the Reconquista, the Christian effort to reclaim Iberia from Moorish rule.

Alfonso received an education befitting a royal prince, including training in warfare, governance, and Christian doctrine. His education prepared him for the military campaigns he would later lead.

Upon the death of his father Ordoño I, Alfonso ascended to the throne at the age of eighteen. His reign would last over four decades and mark the zenith of Asturian power.

Alfonso married Jimena, daughter of García Íñiguez of Pamplona, forging an important alliance with the Basque kingdom. This marriage would produce several sons who would later divide his realm.

Alfonso launched his first major military campaigns, expanding Asturian territory southward. His aggressive military strategy would earn him the epithet 'the Great' and significantly advance the Reconquista.

Alfonso implemented a policy of conquering and repopulating territories south of the Cantabrian Mountains. He established new settlements and fortifications, transforming the demographic landscape of northern Iberia.

Alfonso won a significant victory against Umayyad forces at Polvoraria, demonstrating his military prowess and weakening Muslim control in the border regions.

Alfonso reorganized the administration of his expanding kingdom, establishing new governmental centers and improving the efficiency of royal authority throughout his territories.

Alfonso undertook major construction projects in Oviedo, rebuilding the royal palace and strengthening the city's fortifications. He also enhanced the cathedral and other religious buildings.

Alfonso commissioned the famous Chronicle of Alfonso III, a historical work that traced the Asturian kingdom's origins to the Visigothic monarchy. This chronicle established the ideological foundation of the Reconquista and presented the Asturian kings as legitimate heirs of the Visigoths.

Alfonso achieved a major military victory near Zamora, further pushing the frontier southward and consolidating Christian control over the Duero valley region.

Alfonso became a patron of religious and cultural institutions, founding monasteries and encouraging the preservation of learning. His court became a center of Christian scholarship in Iberia.

Alfonso constructed a network of fortresses along the southern frontier to protect the newly conquered territories from Muslim counterattacks and secure the gains of the Reconquista.

Alfonso began developing León as a major center of his kingdom, recognizing its strategic importance. León would later become the capital of the successor kingdom bearing its name.

Alfonso's sons rebelled against him, forcing his abdication. He divided his kingdom among his three sons: García received León, Ordoño received Galicia, and Fruela received Asturias. This division marked the end of the unified Asturian kingdom.

Alfonso III 'the Great' died shortly after his forced abdication. Despite his tumultuous end, he is remembered as the greatest of the Asturian kings, who expanded Christian territory further than any predecessor and laid the foundation for the Kingdom of León.