Treaty of Windsor Ratified Between England and Portugal
In the late 14th century, Portugal faced threats from Castile during the 1383–1385 succession crisis following the death of King Ferdinand I. John I of Portugal secured victory at the Battle of Aljubarrota in 1385 with English military support, including archers. On May 9, 1386, the Treaty of Windsor was ratified at Windsor, England, formalizing a perpetual alliance of mutual defense and friendship between the Kingdom of Portugal and the Kingdom of England under Richard II. The agreement committed both nations to aid each other against common enemies and was sealed by the marriage of John I to Philippa of Lancaster. This pact built upon earlier treaties from 1373 and established one of history's longest-lasting diplomatic relationships.
Why it matters: The Treaty of Windsor created the oldest alliance still in force today, ensuring Portugal and England (later the United Kingdom) never fought on opposite sides in major conflicts for over six centuries. It provided Portugal with crucial English backing during Iberian power struggles and later supported British interests, including in World War II. The enduring pact exemplifies how medieval diplomacy could foster long-term strategic stability amid shifting European alliances.