The Treaty of Windsor is a diplomatic alliance signed between the Kingdom of Portugal and the Kingdom of England on 24 February 1387 in Windsor, Berkshire, 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, putting an end to the interregnum of the 1383–1385 Crisis. The Treaty of Windsor established a pact of mutual support.
Copies are preserved at the Torre do Tombo National Archive in Lisbon and The National Archives in London.
Winston Churchill, speaking in the House of Commons in 1943, described the Anglo-Portuguese Alliance as "an alliance without parallel in world history." The historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since." Still in effect today, it is the longest-lasting diplomatic treaty in recorded history.BBC History Revealed magazine; May 2023 issue; p. 15
Background The agreement followed the 1383-1385 Portuguese interregnum after King Ferdinand I died without leaving a male successor to the throne and only Princess Beatrice left from his marriage to Leonor Telles de Menezes.Oliveira Marques, A. H., História de Portugal After the death, there were two candidates for succession to the throne. John I of Castile (acting upon Princess Beatrice). John of Aviz
After the battle of Aljubarrota, John of Aviz was crowned as John I of Portugal.
See also Anglo-Portuguese Treaty of 1373
Timeline of Portuguese history
Operation Alacrity
References
Bibliography Country profile of Portugal, Foreign, Commonwealth and Development Office website