John I of Castile and Leon

John I (Spanish: Juan I; 24 August 1358 – 9 October 1390) was King of Castile and León from 1379 until 1390. He was the son of Henry II and Juana Manuel of Castile. John ascended to the throne in 1379, and in 1383, he married Beatrice, the daughter of King Ferdinand I of Portugal. When Ferdinand died that same year, John, aiming to enforce his claim on the Portuguese crown through his wife, brought the country into the 1383–1385 Crisis. His forces faced resistance from Portuguese factions supporting John of Aviz. The conflict culminated in the Battle of Aljubarrota in 1385, where John suffered a defeat, ensuring Portugal's independence.

To secure Castile, he married his son Henry to the daughter of John of Gaunt, linking the Trastámara and Plantagenet dynasties. He died in 1390 when he fell from his horse during a fantasia performance. His death led to minority rule under his son, Henry III.