The Anglo-Portuguese Alliance, 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.
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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:
It is cordially agreed that if, in time to come, one of the kings or his heir shall need the support of the other, or his help, and in order to get such assistance applies to his ally in lawful manner, the ally shall be bound to give aid and succour to the other, so far as he is able (without any deceit, fraud, or pretence) to the extent required by the danger to his ally’s realms, lands, domains, and subjects; and he shall be firmly bound by these present alliances to do this.
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 crusader, 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.
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:
- The War of Spanish Succession, when Portugal together with the Duchy of Savoy initially sided with France, but after the Battle of Blenheim reunited with its ally.
- The Seven Years' War, when Spain invaded Portugal in 1762, Britain intervened on the side of its ally. Although faced with vastly superior numbers, the Portuguese and British forces achieved a military stalemate, which prevented victory for the Spanish and French forces.
- The Napoleonic Wars, when Portugal, isolated in a Europe wholly dominated by Napoleon, continued to trade with the United Kingdom despite French restrictions and was finally invaded, but with British help finally regained total sovereignty and independence. The Portuguese royal family at the time (including the later King John VI) took refuge in its then vice-royalty of Brazil, under escort of the British fleet.
- The Portuguese civil war, when the United Kingdom gave important support to the Liberal faction.
- The 1890 British Ultimatum was considered by Portuguese historians and politicians at that time to be the most outrageous and infamous action of the British against her oldest ally.
20th century 
During the 20th century, the treaty has been invoked several times:
- After German incursions in Portuguese East Africa (Mozambique), Portuguese troops fought on the Western Front during World War I
- In the Second World War, Portugal was neutral but the treaty was invoked by the Allies to establish bases on the Azores
- In 1961, during the Indian invasion of Portuguese India, Portugal sought the help of Britain to little effect
- During the 1982 Falklands War the facilities of the Azores were again offered to the Royal Navy
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.
Historically, the alliance was defining and crucial for determining the borders of Brazil, a large Portuguese-speaking territory in South America, which differentiated itself from its Spanish-speaking neighbors as the United Kingdom took an important role in sponsoring Portugal against Spain in that region.
See also 
- João Ferreira Duarte, The Politics of Non-Translation: A Case Study in Anglo-Portuguese Relations
- Winston Churchill, 12 October 1943 Statement in the House of Commons