Treaty of Windsor (1386)
The Treaty of Windsor is the oldest diplomatic alliance in the world which is still in force. The Anglo-Portuguese Alliance was renewed in 1386 with the Treaty of Windsor and the marriage of King John I of Portugal (House of Aviz) with 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. Recognition from Castile would arrive only in 1411, with the signature of the Treaty of Ayllón. The Treaty of Windsor, which remains valid at the present time, established a pact of mutual support between the countries. This document is preserved at the Portuguese National Archives.
Historical uses of the Treaty 
The two allies (England now part of the United Kingdom) fought together during the Peninsular War (from August 1808 to 1814), where British forces under Sir Arthur Wellesley helped to defend Portugal from French invasion. Having successfully done so, British and Portuguese forces together played an instrumental role in liberating Spain from French occupation.
During the Seven Years' War Portugal and Britain were allies.
In breach of this Alliance, the British government delivered in 1890 an ultimatum to the Portuguese government, forcing it to retreat from part of the central portion of Southern Africa, coveted by Britain, known as the area of the Pink Map (roughly modern Zimbabwe and Zambia). The ultimatum and the subsequent Treaty of London were considered by Portuguese commentators at the time to be an outrageous and infamous action by Britain against her oldest ally.
Portugal fought on the Allied side in World War I in accordance with the Treaty.
During World War II the UK invoked the treaty again to ensure that Portugal would not support the Axis powers, which wasn't really a problem because Prime Minister Salazar was a discreet opponent to these powers and even managed to keep Francoist Spain out of the war. In fact, in 1943 the Portuguese Government leased to Britain what became a major Allied air and naval base in the Portuguese islands, the Azores. Prime Minister Winston Churchill recounted reporting on the lease to the House of Commons:
"I have an announcement", I said, "to make to the House arising out of the treaty signed between this country and Portugal in the year 1373 between His Majesty King Edward III and King Ferdinand and Queen Eleanor of Portugal." I spoke in a level voice, and made a pause to allow the House to take in the date, 1373. As this soaked in there was something like a gasp. I do not suppose any such continuity of relations between two Powers has ever been, or will ever be, set forth in the ordinary day-to-day work of British diplomacy.
See also 
|Wikisource has original text related to this article:|
- "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. Retrieved 4 January 2013.
- João Ferreira Duarte, The Politics of Non-Translation: A Case Study in Anglo-Portuguese Relations
- Parliamentary Debates, House of Commons, 12 October 1943, column 716.
- Winston Churchill, Second World War, pp 146-7