John of Gaunt: Portugal's Kingmaker
John of Gaunt's dynastic ambitions coincided with the urgent need of the Portuguese Crown for foreign support to secure its sovereign independence - the catalyst for a royal marriage and England's oldest alliance.
In July 1386 two men were dining on board ship in Plymouth Sound: the pair were John of Gaunt, Duke of Lancaster, uncle of the reigning Richard II and the most eminent and wealthy noble in England, and his son Henry of Bolingbroke, Earl of Derby, who was to usurp Richard's throne in 1399 as Henry IV. Gaunt set sail with the army with which he was to invade north-west Spain, in the hope of either establishing himself as King of Castile or honourably compromising his claim to its throne.
For several months Gaunt had had his headquarters at Plympton Priory whilst his forces assembled. During his stay in the West Country, on May 9th, a treaty negotiation had been concluded on Richard II's behalf at Windsor Castle with the envoys of the new King of Portugal, Joao (John) I. This was to be a perpetual alliance between the two Kings and their heirs, obliging them to provide each other with military and naval aid on request, to deny assistance to rebels against each other's rule and to give help against attempted overthrows. Stress was also laid on the protection to be given to traders. Gaunt, in his role as pretender to the Castilian throne, had just made a perpetual peace between Castile and Portugal. Portuguese ships formed part of his invasion fleet: once he established his rule on Spanish soil, in parts of Galicia, co-operation between him and João blossomed.