|King of Asturias|
|Successor||Fruela II (Asturias)|
García I (León)
Ordoño II (Galicia)
|Died||20 December 910|
|Consort||Jimena of Pamplona|
|Issue||Fruela II of León|
García I of León
Ordoño II of León
|Father||Ordoño I of Asturias|
Alfonso III (c. 848 – 20 December 910), called the Great (Spanish: el Magno), was the king of León, Galicia and Asturias from 866 until his death. He was the son and successor of Ordoño I. In later sources he is the earliest to be called "Emperor of Spain." He was also titled "Prince of all Galicia" (Princeps totius Galletiae).
Alfonso's reign was notable for his comparative success in consolidating the kingdom during the weakness of the Umayyad princes of Córdoba. He fought against and gained numerous victories over the Muslims of al-Andalus.
During the first year of his reign, he had to contend with a usurper, Count Fruela of Galicia. He was forced flee to Castile, but after a few months Fruela was assassinated and Alfonso returned to Oviedo.
He defeated a Basque rebellion in 867 and, much later, a Galician one as well. He conquered Oporto and Coimbra in 868 and 878 respectively. In about 869, he formed an alliance with the Kingdom of Pamplona, and solidified this link by marrying Jimena, who is thought to have been daughter of king García Íñiguez, or less likely, a member of the Jiménez dynasty, and also married his sister Leodegundia to a prince of Pamplona.
The following year, 867, Alfonso had to attend to an uprising in the eastern part of the kingdom, in Alava, vascones according to the Chronicle of Albeda, Alava according to the Chronicle of Sampiro that had revolt led by Count Eylo.  Sampiro describes These events:
A messenger arrived from Álava, announcing that their hearts had inflated against the king: hearing that, the monarch decided to march there. Driven by the fear of their arrival, they quickly recognized their obligations and supplicants, lowered their heads before him and promised that they would remain faithful to his kingdom and authority, and that they would do what was commanded. In this way he submitted to his power an Alava lying before him, and Eylo, who presented himself as his count, brought him to Oviedo loaded with iron.Template:Harvnp
His father, Ordoño, had begun the repopulation of the border territories and Alfonso continued with it. The first successes were harvested in Portuguese lands, where King Alfonso's troops succeeded in locating the southwestern frontier on the Mondego river. Count Vimara Perez in 868 conquered Oporto and repopulated the district.  In 878, the army of King Alfonso III, with Count Hermenegildo Gutiérrez in command, faced the Muslim forces led by the emir of Cordoba Mohammad I who they had started an attack against Oporto.  [a] After defeating the emir's forces and expelling the Muslim inhabitants of Coimbra and Oporto, from both cities the Christian troops, led by Hermenegildo, occupied and repopulated, with men taken from Galicia, other cities such as Braga, Viseo and Lamego.  Coimbra, Lamego and Viseo were conquered again in 987 by Almanzor  and it was not until 1064 when they were finally reconquered by King Ferdinand I of León. 
Alfonso III had to face the offensive of the Umayyad prince al-Mundir, son of the Cordovan emir Mohamed I. The fight was almost constant between 875 and 883. The first Umayyad raids were aimed León and El Bierzo, but failed. The Christian counteroffensive was settled with the taking of Deza and Atienza.
Abd al-Rahman Ibn Marwan, the Galician, Lord of Mérida and rebel to the Emir of Córdoba, sent him to ingratiate himself with him to the Minister of this, Hashim ibn Abd al-Aziz.  This caused that in 878 Al-Mundir he directed his hosts back to Leon and Astorga, while Salid ben Ganim reached the Órbigo. The great king, to avoid the union of both armies, went out to meet the second he defeated in the battle of Polvoraria, at the confluence of the Órbigo and Esla rivers. Al-Mundir then undertook the withdrawal, but Alfonso III intercepted him in the valley of Valdemora, where he defeated him. Emir Mohamed was forced to pay ransom and sign a three-year truce. It was the first time that Córdoba asked for peace. 
Both kings considered the truce as a parenthesis, preparing for the next assault: Mohamed armed a fleet to attack Galicia by sea, but was destroyed by a storm.  Alfonso and Ibn Marwan descended through the Tagus Valley and defeated the army Cordovan on Mount Oxifer, next to the Guadiana River. 
As revenge, Mohamed attacked in the year 882 the kingdom of Zaragoza, where Alfonso had sent his son Ordoño to be educated with the Banu Qasi, sons of Musa, advancing through the ancient Roman road to Leon. There was an exchange of prisoners and the Cordovans withdrew. They repeated the campaign in 883 with the same result. In 884 Mohamed I and Alfonso III signed the peace, since both began to have serious internal problems. The great king was met with a rising led by his brothers Fruela, Odoario and Bermudo, who became strong in Astorga, supported by several counts, but were quickly defeated and executed. In 901 the Umayyad rebel Ibn al-Qitt, proclaimed Mahdi, preached holy war and attacked Zamora - "rebuilt and repopulated by Mozarabic Toledo [...] the most important advanced square of the Asturian kingdom" - which he was able to resist. The messianic leader, abandoned by his own, was defeated and killed in battle on what is known as the Day of Zamora.  In those years, the emirate of Cordoba suffered civil rebellions, so it stopped disturbing the kingdom of Asturias, however, faced his former allies of Mérida and the Ebro valley: allied with the Count of Pallars, gave a coup that managed to defeat the Banu Qasi and install a Navarre, Sancho Garcés I, in the throne of Pamplona.
He ordered the creation of three chronicles which presented the theory that the kingdom of Asturias was the rightful successor of the old Visigothic kingdom. He was also a patron of the arts, like his grandfather before him. He built the church of Santo Adriano de Tuñón. According to a letter of disputed authenticity dated to 906, the Epistola Adefonsi Hispaniae regis, Alfonso arranged to purchase an "imperial crown" from the cathedral of Tours.
In 909, Alfonso relocated the seat of his government to Oviedo. According to Sampiro, his sons (García, Ordoño, Gonzalo, Fruela and Ramiro) conspired against him, under the influence of García's father-in-law. Alfonso had García imprisoned but they were able to eject him and he fled to Boiges. However, he returned and convinced García to join him in a campaign against the Moors. Alfonso died in Zamora of natural causes in 910, having reigned for 44 years. Ibn Hayyan likewise tells of an uprising, but says that Alfonso himself had been imprisoned. Following his death there was a partition of his realm: his eldest son, García, became king of León. The second son, Ordoño, reigned in Galicia, while Fruela, received Asturias with Oviedo as his capital. These lands would be reunited when García died childless and León passed to Ordoño, while on his death the lands were reunited under Fruela. However, Fruela's death the next year initiated a series of internecine struggles that led to unstable succession for over a century.