Anglo - Portuguese Alliance The Anglo - Portuguese Alliance ( or Aliança Luso - Britânica ) , ratified at the Treaty of Windsor in 1386 , between England ( succeeded by the United Kingdom ) and Portugal is the oldest alliance in the world which 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 . History . Middle Ages . English aid to the House of Aviz set the stage for the cooperation with England that would be the cornerstone of Portuguese 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 . In May 1386 , the Treaty of Windsor sealed the alliance that first started in 1294 , and was confirmed at Aljubarrota with a pact of perpetual friendship between the two countries . The most important part of the treaty stated that : The next year , John of Gaunt , Duke of Lancaster , son of king Edward III of England , and father of king Henry IV of England , landed in Galicia with an expeditionary force to press his claim to the Crown of Castile with Portuguese aid . He failed to win the support of the Castilian nobility and returned to England with a cash compensation from the rival claimant . John of Gaunt left behind his daughter , Philippa of Lancaster , to marry king John I of Portugal in order to seal the Anglo - Portuguese alliance . By this marriage , celebrated in 1387 , John I became the father of a generation of princes called by the poet Luís de Camões , the " marvellous generation " , who led Portugal into its golden age , during the period of the Discoveries . Philippa brought to the court the Anglo - Norman tradition of an aristocratic education and gave her children good educations . Her personal qualities were the highest , and she reformed the court and imposed rigid standards of moral behaviour . On the other hand , her method was also seen as too traditional or outdated by the more tolerant Portuguese aristocracy . Philippa provided royal patronage for English commercial interests that sought to meet the Portuguese desire for cod and cloth in return for wine , cork , salt , and oil shipped through the English warehouses at Porto . Her eldest son , Duarte , authored moral works and became king in 1433 ; Pedro , who travelled widely and had an interest in history , became regent when Duarte died of the plague in 1438 ; Fernando – Ferdinand the Saint Prince – who became a crusade r , participated in the attack on Tangiers in 1437 ; and Henrique – Prince Henry the Navigator – became the master of the Order of Christ and the instigator and organiser of the early voyages of discovery . Disruption . The Iberian Union ( 1580 – 1640 ) , a 60 year dynastic union between Portugal and Spain , interrupted the alliance . Portuguese foreign policy became tied to Spanish hostility to England . As a result , Portugal and England were on opposite sides of the Anglo - Spanish War ( 1585 – 1604 ) and the Dutch - Portuguese War . The alliance was reconfirmed after the Portuguese Restoration War and the English Restoration . 17th to 19th Centuries . The alliance was reconfirmed after the breakup of the Iberian Union , primarily due to both countries ' respective rivalries with Spain , the Netherlands , and France , both in Europe and overseas . During this time , important episodes in the alliance were : 20th century . During the 20th century , the treaty has been invoked several times : Importance in modern times . Today , as both countries are members of the European Union and NATO , their relations are largely coordinated through those institutions rather than by the provisions of the many treaties forming the Anglo - Portuguese Alliance .