Dated 10 July 1372, “Treaty of Tagilde” was a document recording the intention of King Fernando I of Portugal and John of Gaunt, Duke of Lancaster, to become allies in their disputes with the nascent state of Spain. Soon afterwards, French and Castilian warships defeated the English at the battle of La Rochelle and blockaded Lisbon. Then a Castilian army was marched via Viseu to take Coimbra . Because Lancaster failed to send the promised troops Fernando was forced to sign on 19 March 1373 at Santarém a humiliating peace which obliged Portugal to become an enemy of England, provide galleys to augment the Castilian navy and to give hostages.
However Fernando (aptly named The Inconsistent) welshed on the Santarém treaty, which was intended to last for a term of seven years, and concluded the first Anglo-Portuguese Alliance in St. Paul´s Cathedral on 16 June 1373. His duplicity in signing conflicting treaties was rewarded by ten years of chaos during which his subjects came close to civil war with the merchant class advocating full independence while the nobility, including the king´s two half brothers, favoured unity with Castile. As the king´s health deteriorated plot and counterplot followed until the Count of Ourem arranged for a rabble of 3,000 English soldiers/ mercenaries to land in Portugal under the ill-advised leadership of the Duke of Cambridge. They terrorised the frontier townships causing great distress to the citizenry and were eventually sent packing while a reduced Portuguese army and militias faced a large Castilian army near Badajoz. However, battle was avoided by a secret agreement whereby Fernando´s daughter, Beatrix, would marry the younger son of King Juan.
Into the breach caused by this morass stepped John of Avis. He commenced a reign of 43 years on 06 April 1384 and his strength of leadership brought about national unity and led to a comprehensive Treaty of political , economic and military co-operation being signed at Windsor in May 1386 ; it is from this date that commemorations should be calculated.
The successes and failures of the Windsor Treaty were detailed in the six part series of my essays concerning the Anglo-Portuguese alliance which you kindly published in 2021.