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The Treaty of Windsor was a diplomatic alliance signed between Portugal and England on May 9, 1386, in Windsor and sealed by the marriage of King John I of Portugal (House of Aviz) to Philippa of Lancaster , daughter of John of Gaunt, 1st Duke of Lancaster. With the victory at the Battle of Aljubarrota, assisted by English archers, John I was recognised as the undisputed King of Portugal, putti...