King of Castile (1157–1158), whose one-year reign began the separate Castilian kingdom, founded the Order of Calatrava, and left an infant heir in Alfonso VIII.
Sancho III, called "the Desired", was king of Castile for a single year, from 1157 to 1158. The elder son of Alfonso VII, emperor of León and Castile, he inherited Castile when his father, on his deathbed, divided his realms between his two sons — giving Castile to Sancho and León to his brother Ferdinand II, and so beginning the long separation of the two Spanish kingdoms.
Sancho's brief reign was notable chiefly for the founding of the Order of Calatrava, the first Spanish military order, established to defend the frontier fortress of Calatrava against the Almohads. He died suddenly in 1158, leaving as his heir the infant Alfonso VIII, whose long minority would plunge Castile into a bitter struggle for the regency.
Sancho was born in 1134, the elder son of Alfonso VII, who styled himself emperor of Spain and ruled both León and Castile. Raised as heir to a great realm, Sancho found his inheritance divided when Alfonso VII, following Iberian custom, split his dominions between his sons at his death in 1157: Sancho received Castile, and his younger brother Ferdinand received León.
Sancho III reigned too briefly to leave a rounded portrait, but the sources present him warmly, and his byname "the Desired" reflects the affection and high hopes attached to him. He appears as a promising and well-regarded young king, pious and active, whose short rule was cut off before it could show its full measure.
The one enduring act of his reign — his patronage of the new Order of Calatrava — reveals a king alert to the central task of Castilian kingship, the defence and extension of the Christian frontier against the Muslim south. In entrusting an exposed fortress to a brotherhood of warrior-monks, he showed both piety and strategic sense. He is remembered as the good king who died too soon, leaving his kingdom to the perils of a royal minority.
Sancho's single year on the throne is remembered above all for the founding of the Order of Calatrava in 1158. When the Templars declared they could no longer hold the frontier fortress of Calatrava against the threatened Almohad advance, Sancho granted it to an abbot and his followers, who formed a new military-religious order to defend it — the first of the great Spanish orders, and a lasting instrument of the Reconquista.
Beyond this, Sancho worked to secure his new and separate kingdom of Castile and to manage relations with his brother in León, but he had little time to shape his reign before it ended.
Sancho III died on 31 August 1158, after a reign of barely a year, and was buried at Toledo. His heir was his son Alfonso VIII, a child of about three, whose minority at once became the prize in a violent contest between the great noble houses of Castile.
Sancho III is remembered as the king whose brief reign founded the independent Kingdom of Castile and the Order of Calatrava. The division of his father's empire that made him king began the long separation of Castile and León that would last, with interruptions, until 1230. And though his own reign was short, his son Alfonso VIII would grow to be one of the greatest of Castilian kings, the victor of Las Navas de Tolosa.