Treaty of Windsor (1386)

The Treaty of Windsor is a diplomatic alliance between the Kingdom of Portugal and the Kingdom of England, signed on 24 February 1387 at Windsor in Berkshire and sealed by the marriage of King John I of Portugal, of the House of Aviz, to Philippa of Lancaster, daughter of John of Gaunt. After the victory at the Battle of Aljubarrota, won with the help of English archers, John I was recognised as the undisputed King of Portugal, ending the interregnum of the 1383 to 1385 crisis, and the treaty established a pact of mutual support between the two crowns. Copies survive in the Torre do Tombo National Archive in Lisbon and at The National Archives in London. Winston Churchill described the Anglo-Portuguese Alliance in the House of Commons in 1943 as an alliance without parallel in world history, and the treaty, still in effect today, is described as the longest-lasting diplomatic treaty in recorded history.