Peter I (Peter the Cruel or the Just) (b. 1334, Burgos, Castile, d. 1369, Montiel, France), celebrated king of Castile and Leon from 1350 to 1369, charged by his contemporary enemies with monstrous cruelty but viewed by later writers as a strong executor of justice.
He succeeded his father, Alfonso XI, at the age of 15, and John II of France saw the chance to force Castile into a military alliance against England. The alliance was concluded (1352), and Peter was forced to marry (1353) Blanche, daughter of Pierre, duc de Bourbon, though he was already passionately in love with the beautiful María de Padilla - who was to remain his mistress, and perhaps his legal wife, until her death (1361). He abandoned Blanche immediately after the marriage. This act ruptured the Franco-Castilian alliance.
At home Peter was at once confronted by a row of bastard half brothers, led by Henry of Trastámara (later Henry II), who, to win support for his undefined ambitions, proclaimed himself defender of the magnates' privileges against the growing power of the crown. After leading several revolts which Peter crushed with energy, Henry, who failed to win any popular sympathy, escaped to France (1356) and offered to serve the French crown against his brother. From 1356 to 1366 Peter was engaged in a bitter war with Aragon, whose king, Peter IV, supported Henry's cause. During the war Peter won many successes against Aragon while Trastamaran propaganda failed to undermine Castilian loyalty toward him. In 1365, therefore, the French king Charles V, Pope Urban V, and Peter IV - to save Aragon from being overrun - paid veteran French mercenaries, led by Bertrand du Guesclin, to go to Spain and overthrow Peter, replacing him by Henry. Peter fled to Gascony and requested English help under the Anglo-Castilian alliance, concluded on June 22, 1362. The Trastamarans and their French allies were routed at Nájera (April 3, 1367) by Edward the Black Prince, and Peter resumed his reign.
Charles V sent Henry back to Spain with more French troops and a long civil war ensued. Eventually Peter was defeated at Montiel and assassinated there by his brother's own hand.