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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 and anarchy of the 1383–1385 Crisis. Recognition from Castile would arrive only in 1411, with the signature of the Treaty of Ayton-Segovia. The treaty, which remains valid at the present time, established a pact of mutual support between the countries.

Historical uses of the Treaty

Portugal used it against its neighbours in 1640, to expel the Spanish kings (House of Habsburg) from the country.

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 fierce allies.

In breach of this Alliance, the British government delivered in 1890 an ultimatum to the Portuguese government, forcing it to retreat from that portion of Africa, coveted by Britain, known as the area of the Pink Map.

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, and in 1943 the Portuguese dictator, Salazar, leased to Britain what became a major Allied air and naval base in the Portuguese islands of 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 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.[1]

In a breach of the Alliance, when, in 1961, the Portuguese provinces of Goa, Daman and Diu (Portuguese India) were under assault by Jawaharlal Nehru's Indian Union army in the Invasion of Goa, Portugal requested the assistance of the United Kingdom, but it fell on deaf ears.


  1. ^ Winston Churchill, Second World War, pp 146-7

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