Anglo-Portuguese Alliance (or Aliança Luso-Britânica), ratified at the Treaty of Windsor in 1386, between England (succeeded by the United Kingdom) and Portugal (Lusitania) is the oldest alliance in the world that is still in force — with the earliest treaty dating back to the Anglo-Portuguese Treaty of 1373.

This alliance, which goes back to the Middle Ages, has served both countries. It was very important throughout history, influencing the participation of the United Kingdom in the Iberian Peninsular War, the UK’s major land contribution to the Napoleonic Wars and the establishment of an Anglo-American base in Portugal. Portugal aided England (and later the UK) in times of need, for example, in the First World War.

John of Gaunt being entertained by John I of Portugal.

English aid to the House of Aviz (which ruled Portugal from 1385 to 1580) set the stage for Portuguese cooperation with England that would become a cornerstone of Portugal’s foreign policy for more than five hundred years. However, English aid to Portugal went back much further to the 1147 Siege of Lisbon, when English and other Northern European Crusaders – en route to the Holy Land to participate in the Second Crusade – stopped and helped Portuguese King Afonso Henriques to conquer the city from the Moors. In May 1386, the Treaty of Windsor sealed the alliance – first started in 1294, renewed in the Anglo-Portuguese Treaty of 1373 and confirmed at the Battle of Aljubarrota (1385) – with a pact of perpetual friendship between the two countries. The most important part of the treaty stated that:

During the 20th century, the treaty has been invoked several times:

First World War

• After German incursions in Portuguese East Africa (today Mozambique), Portuguese troops fought on the Western Front alongside Allied soldiers during the First World War. Brazil also supported the Allies.

Second World War

Upon the declaration of war in September 1939, the Portuguese Government announced on 1 September that the Anglo-Portuguese Alliance remained intact, but since the British did not seek Portuguese assistance, Portugal would remain neutral. In an aide-mémoire of 5 September 1939, the British Government confirmed the understanding. British strategists regarded Portuguese non-belligerency as “essential to keep Spain from entering the war on the side of the Axis

• During the Second World War, Salazar steered Portugal down a middle path, but nevertheless provided aid to the Allies. The British Ambassador in Lisbon, Ronald Campbell, saw Salazar as fundamentally loyal to the Alliance and stated that “he [Salazar] would answer the call if it were made on grounds of dire necessity”. When, in August 1943, the British requested base facilities in the Azores and invoked the alliance that had existed for over 600 years between Portugal and Great Britain, Salazar responded favourably and virtually at once: Portugal granted naval bases on Portuguese territory to Britain, in keeping with the traditional Anglo-Portuguese Alliance, letting them use the Azorean ports of Horta (on the island of Faial) and Ponta Delgada (on the island of São Miguel), and the airfields of Lajes Field (on Terceira island) and Santana Field (on São Miguel Island).
• In November 1943, the British Ambassador in Lisbon, Sir Ronald Campbell, wrote (paraphrasing Salazar) that “strict neutrality was the price the allies paid for strategic benefits accruing from Portugal’s neutrality and that if her neutrality instead of being strict had been more benevolent in our favour Spain would inevitably have thrown herself body and soul into the arms of Germany. If this had happened the Peninsula would have been occupied and then North Africa, with the result that the whole course of the war would have been altered to the advantage of the Axis.”
• From November 1943, when the British gained the use of the Azores, to June 1945, 8,689 U.S. aircraft departed from Lajes base in the Azores, including 1,200 B-17 and B-24 bomber aircraft were ferried across the Atlantic. Cargo aircraft carried vital personnel and equipment to North Africa, to the United Kingdom and – after the Allies gained a foothold in Western Europe – to Orly Field near Paris. Flights returning from Europe carried wounded servicemen. Medical personnel at Lajes, Azores, handled approximately 30,000 air evacuations en route to the United States for medical care and rehabilitation. By using Lajes Field, it was possible to reduce flying time between the United States and North Africa from 70 hours to 40. This considerable reduction in flying hours enabled aircraft to make almost twice as many crossings per month between the United States and North Africa and demonstrated clearly the geographic value of the Azores during the war.

Postwar

• The establishment of EFTA, a free trade area created under the sphere of influence of the United Kingdom, took place in 1959, at the same time of the establishment of the European Economic Community. Portugal was one of the founding members. The EFTA (European Free Trade Association) was established by the European countries that did not wish to join the EEC: Great Britain, Austria, Denmark, Portugal, Norway, Sweden and Switzerland. In 1973, Great Britain abandoned the organization, which was followed later by Portugal and Denmark when they joined the EEC.
• In 1961, during the Indian invasion of Portuguese India, Portugal sought the help of Britain.
• During the 1982 Falklands War, the facilities of the Azores were again offered to the Royal Navy.