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, assisted by English archers, 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.
Historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since."
- Livermore 1947, p. 179.
- "Tratado de paz, amizade e confederação entre D. João I e Eduardo II, rei de Inglaterra, denominado Tratado de Windsor" (in Portuguese). Portuguese National Archives Digital Collection. http://digitarq.dgarq.gov.pt/details?id=4186035. Retrieved 4 January 2013.
- Winslett 2008, p. 3.
- Livermore, H.V (1947). A History of Portugal. Cambridge University Press. 179
- Winslett, Matthew (2008). The Nadir of Alliance: The British Ultimatum of 1890 and Its Place in Anglo-Portuguese Relations, 1147--1945. University of Texas Arlington. https://uta-ir.tdl.org/uta-ir/handle/10106/1093. 3
- Country profile of Portugal, Foreign and Commonwealth Office website
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