English aid to the House of Avis set the stage for the cooperation with England that would be the cornerstone of Portuguese foreign policy for more than 500 years. In May 1386, the Treaty of Windsor confirmed the alliance that was born at Aljubarrota with a pact of perpetual friendship between the two countries. The next year, John of Gaunt, duke of Lancaster, son of Edward III, and father of Henry IV, landed in Galicia with an expeditionary force to press his claim to the Castilian throne 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 João I in order to seal the Anglo-Portuguese alliance. By this marriage, celebrated in 1387, João became the father of a generation of princes called by the poet, Luís de Camões, the "marvelous generation," who led Portugal into its golden age. Philippa brought to the court the Anglo-Norman tradition of an aristocratic education and gave her children good educations. Her personal qualities were of the highest, and she reformed the court and imposed rigid standards of moral behavior. 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 traveled widely and had an interest in history, became regent when Duarte died of the plague in 1438 Fernando, 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 Avis and the instigator and organizer of the early voyages of discovery.

Data as of January 1993