John I of Portugal was the King of Portugal from 1385 until his death in 1433. He was a member of the Aviz dynasty and is best known for his role in establishing Portuguese independence from Castile after the decisive victory at the Battle of Aljubarrota in 1385. His reign marked the beginning of a period of exploration and expansion for Portugal. John I was also notable for his marriage to Philippa of Lancaster, which strengthened ties between Portugal and England. This alliance led to the Anglo-Portuguese Treaty, one of the oldest military alliances still in effect today. His legacy includes the promotion of trade and exploration, setting the stage for Portugal's Age of Discovery.