Alfonso III, called the Great, 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".

The Kingdom of León was an independent kingdom situated in the northwest region of the Iberian Peninsula. It was founded in 910 when the Christian princes of Asturias along the northern coast of the peninsula shifted their capital from Oviedo to the city of León. The kings of León fought civil wars, wars against neighbouring kingdoms, and campaigns to repel invasions by both the Moors and the Vikings, all in order to protect their kingdom's changing fortunes.