Treaty of Windsor (1386)

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.[1] 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.[1] The Treaty of Windsor established a pact of mutual support between the countries.[1]

Treaty of Windsor

Treaty of peace, friendship and confederation between John I of Portugal and Richard II, King of England
Treaty of Windsor
Drafted	9 May 1386
Signed	24 February 1387
Location	Westminster
Negotiators	Fernando Afonso de Albuquerque Lourenço João Fogaça Richard d'Alberbury John Clanowe Richard Ronhale
Signatories	Kingdom of England  Kingdom of Portugal
Ratifiers	Richard II of England
Depositary	Portuguese National Archives
Language	Latin

Marriage of John I, King of Portugal and Philippa of Lancaster, daughter of John of Gaunt, 1st Duke of Lancaster.

This document is preserved at the Portuguese National Archives.[2]

Historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since."[3]

Enduring to this day, the Treaty of Windsor holds the record as the longest-lasting diplomatic treaty in recorded history.[4]