The Treaty of Windsor is the diplomatic alliance signed between Portugal and England on 9 May 1386 at 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, John I was recognized 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 between the countries. This document is preserved at the Portuguese National Archives.