The 650th anniversary of this peace treaty will be commemorated on 19th March. It marked the ending of the second war between the kings Fernando I of Portugal and Enrique II of Castile and was cemented by two royal marriages : Princess Beatriz, the sister of Fernando I and daughter of king Pedro of Portugal by D. Ines de Castro was to marry Sancho Alfonso of Castile while the illegitimate daughter of Fernando I , Isabel, was to wed Count Alfonso of Noreña and Gijon a son of the Castilian monarch.

A requirement of the Treaty was that all disloyal Castilians should be banished from the realm. It also demanded the end of English commercial and military interests in Portugal and the cancellation of the Treaty of Tagilde which had been signed in July 1372 by John of Gaunt, duke of Lancaster. He had promised to bring troop reinforcements from England in return for Portuguese support of his claim to the throne of Castile due to his marriage to Constanza, the daughter of Pedro I. But the defeat of the English navy by the French at a sea battle near La Rochelle and the blockade of Lisbon and Porto by Castilian warships thwarted Lancaster´s aims. Following a deep invasion into Portugal by Castilian expeditionary forces, Fernando was brought to his knees.

This rather fragile peace lasted for the intended seven year duration of the Treaty despite opposition from the Portuguese people which included the seizure of the castle of Ourém by intended hostages. However, only three months later a treaty of alliance between England and Portugal was signed in London by king Edward III and ratified by Fernando I (The Inconstant). The discovery of this perfidy brought little reaction from the Castilian regime which had placed its own administrators in the Portuguese economy and was confidant that its nautical hegemony would prevent any practical observance by the English of their military obligations. The trade established by English merchants through their unofficial Treaty of 1353 was allowed to continue as it was considered to be beneficial to both Iberian economies.

This status quo continued until 1381 when João Andeiro , then the consort of Queen Leonor (Teles de Menezes), revived the 1373 Treaty by inviting the duke of Cambridge to lead an English rabble of mercenaries which plundered and caused grievous bodily harm to both Castilians and Portuguese who lived in the frontier zones. By joint action, the English marauders were expelled by Portuguese and Castilians who agreed terms for a peace founded, yet again, upon marriage of royalty and nobility.

The quango , Portugal-UK 650, which is now preparing for celebrations in July of the London Treaty would do well to incorporate references to the consequences of the preceding Treaty of Santarém.