John I (John the Great), 1357?–1433, king of Portugal (1385–1433), illegitimate son of Peter I. He was made (1364) grand master of the Knights of Aviz and exercised his influence in opposition to Leonor Teles, the queen of his half-brother, Ferdinand I. After Ferdinand's death (1383), his widow and her lover, the conde de Ourém, set up a regency in the name of Ferdinand's daughter Beatrice, wife of John I of Castile. This provoked a popular national revolt, led by John of Aviz, who murdered Ourém, and Nun' Álvares Pereira. The Castilians invaded (1384) Portugal, but their forces were decimated by the plague while they laid siege to Lisbon. John was elected king in 1385, and in the same year a great victory over the Castilians at Aljubarrota assured Portuguese independence (though peace was not finally concluded until 1411). John's position was strengthened by an alliance with England, sealed by a treaty (1386) and by John's marriage (1387) to Philippa, daughter of John of Gaunt. The reign of John the Great was one of the most glorious in medieval Portuguese history. His popularity was heightened by his administrative reforms. His sons, Duarte, Peter, Henry the Navigator, John, and Ferdinand, were important in inaugurating the era of Portuguese colonial and maritime expansion. Ceuta in N Africa was conquered from the Moors in 1415. John was succeeded by his son Duarte.
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