John I, 1358–90, Spanish king of Castile and León (1379–90), son and successor of Henry II. He tried unsuccessfully to unite the Portuguese and Castilian crowns but was twice defeated by the Portuguese, notably in the battle of Aljubarrota (1385). He defended his crown against John of Gaunt and married his son Henry to John of Gaunt's daughter. Henry succeeded him as Henry III.
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