The Treaty of Windsor, signed on May the 9th of 1386 between Portugal and England, represents the oldest diplomatic alliance still in force in the world. It arose in a context of political instability in Portugal, following the crisis of 1383-1385 and the Portuguese victory at the Battle of Aljubarrota, which confirmed D. João I as king and secured national independence from Castile (Spain). The alliance with the English crown was a way to secure external support and consolidate the new monarch’s position.
The treaty established a commitment of “friendship, mutual cooperation, military support, and freedom of movement and trade” between the subjects of the two kingdoms and it can even be seen as a “prototype” of the foundations that would come to define the European Union and its free movement of people and goods. The alliance was quickly strengthened with the marriage between D. João I and Philippa of Lancaster (Filipa de Lencastre), daughter of John of Gaunt, Duke of Lancaster, which cemented the ties between the two crowns and gave rise to a generation, with notable figures such as Infante D. Henrique, O Navegador (Prince Henry the Navigator). Over the centuries this relationship has withstood the test of time, political change, and international challenges. During the Restoration of Portuguese Independence in 1640, England supported Portugal in the face of the Spanish threat. In the 18th century, the Treaty of Methuen (1703) strengthened economic relations, promoting the export of Portuguese wines to England and English textiles to Portugal, and in the 19th century, during the Napoleonic Invasions, British military support was crucial in the defence of Portuguese territory.
Even with occasional disagreements, such as the Portuguese neutrality in World War II, the alliance remained stable and active. Today, the Portuguese-British relationship is expressed in diplomatic, commercial, cultural, and human ties, with thousands of British residents in Portugal and close cooperation in various fields. The enduring friendship between the two countries is a rare symbol of historical continuity and mutual respect
