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The Anglo-Portuguese Alliance (or Aliança Luso-Inglesa, "Luso-English Alliance") is the oldest military alliance that is still in force by political bilateral agreement. It was established by the Treaty of Windsor in 1386, between the Kingdom of England (since succeeded by the United Kingdom) and the Kingdom of Portugal (now the Portuguese Republic), though the countries were previously allied via the Anglo-Portuguese Treaty of 1373.
Historically, the Kingdom of Portugal and the Kingdom of England, and later the modern Portuguese Republic and United Kingdom, have never waged war against each other nor have they participated in wars on opposite sides as independent states since the signing of the Treaty of Windsor. While Portugal was subsumed under the Iberian Union, rebellious Portuguese factions and government in exile sought refuge and help in England. England spearheaded the Anglo-Spanish War (1585–1604) on the side of the deposed Portuguese royal house.
The alliance has served both countries throughout their respective military histories, influencing the participation of the United Kingdom in the 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. Today, Portugal and the United Kingdom are both part of NATO, a larger intergovernmental military alliance between several North American and European states that accounts for over 70% of total global military spending.
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 Treaty of Tagilde in 1372 and the ensuing 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:
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.
In July 1386, John of Gaunt, Duke of Lancaster, son of the late king Edward III of England and father of the future 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 (February 1387) in order to seal the Anglo-Portuguese alliance. By this marriage, John I became the father of a generation of princes called by the poet Luís de Camões the "Illustrious Generation", which led Portugal into its golden age, during the period of the Discoveries.
Philippa brought to the court the Anglo-Norman tradition of an aristocratic upbringing and gave her children good educations. Her personal qualities were of the highest standard, and she reformed the court and imposed rigid standards of moral behaviour. On the other hand, the more tolerant Portuguese aristocracy saw her methods as too traditional or outdated.
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 (1439–1448) after Duarte died of the plague in 1438; Ferdinand the Saint Prince (1402–1443), who became a crusader, participated in the attack on Tangiers in 1437; and Henrique – also known as Prince Henry the Navigator (1394–1460) – became the master of the Order of Christ and the instigator and organiser of Portugal's early voyages of discovery.
The Iberian Union (1580–1640), a 60-year dynastic union between Portugal and Spain, interrupted the alliance. The struggle of Elizabeth I of England against Philip II of Spain in the sixteenth century meant that Portugal and England were on opposite sides of the Anglo-Spanish War (1585–1604) and the Dutch–Portuguese War. Portuguese foreign policy became tied to Spanish hostility to England. England also captured the Portuguese garrison of Ormuz in Persia in 1622.
However, in 1640, England supported the Portuguese House of Braganza to take power in Portugal replacing the House of Habsburg, putting an end to a 60-year dynastic union between Portugal and Spain. England's support for Portugal during their Restoration War was confirmation of the renewal of the alliance. This was solidified further after the English Restoration and the marriage of Catherine of Braganza and Charles II of England. Portugal ceded Tangier and Bombay as part of the dowry. England in addition to military support on the ground would protect Portuguese shipments in the Mediterranean and the coasts of Lisbon and Porto. Following the defeat of Spain in the war, England mediated the Treaty of Lisbon in 1668 which saw the independence of Portugal and the recognition of Pedro II as King. The English alliance was decisive in the consolidation of the independence of Portugal, and in Pedro's leadership. In return Portugal promised to transfer to the English the majority of the places recovered from the Dutch, to share in half the commerce of cinnamon and to install English families with the same privileges as Portuguese families in Goa, Cochin, Diu, Bahia, Pernambuco and Rio de Janeiro.
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:
During the 20th century, the treaty was invoked several times:
On 13 June 2022, the Prime Minister of Portugal and the Prime Minister of the United Kingdom signed a new agreement between the two nations in London, known as the UK-Portugal Joint Declaration on Bilateral Cooperation, thereby reinforcing the Anglo-Portuguese Alliance and confirming its status as the longest-running alliance still in force. The Joint Declaration was also signed to celebrate the 650th anniversary of the Treaty of Tagilde.  However, as both countries are members of NATO, their relations are now largely coordinated through that institution, rather than by the bilateral treaty.
This article incorporates text from this source, which is in the public domain. Country Studies. Federal Research Division.