OTD MAY 9TH: THE OLDEST PEACE TREATY IN THE WORLD IS AGREED

A medieval alliance made in the age of John of Gaunt has been in force ever since the 1380s

In the history of war and diplomacy, May 8th/9th has one major association. Depending on where you live in the world, one of those two days is the anniversary of the moment in 1945 when representatives of Nazi Germany signed an instrument of surrender, ending the war in Europe. In the west we mark this as VE Day, on the 8th. In Russia (as we are all too aware at present) it’s known as Victory Day, and observed on the 9th.

Yet May 9th has a medieval connection, too: it’s the anniversary of the agreement of the Treaty of Windsor of 1386. And though not many people talk about this treaty any more, it’s worth remembering the name. Because that pact of friendship and alliance between England and Portugal has the distinction of being the oldest to remain continuous and unbroken in history.

The Treaty of Windsor, today held in the National Archives

The background was a war. Between the 1340s and 1380s the Hundred Years War had spread from a relatively contained conflict between England and France to spawn theatres of conflict in Scotland, Flanders and the Iberian Peninsula. In 1373, Edward III of England had made a formal military alliance with the Portuguese. In 1384/5 the English were called upon to honour this and help Portugal resist an invasion by John of Trastámara, king of Castile.

By this point Edward III was dead, but the government of his grandson, Richard II, was persuaded. Several hundred veteran English archers were deployed to assist with the defence of Portugal. They played a critical role at the battle of Aljubarrota (August 14th 1385) in which the Portuguese under Joao, grand master of the Order of Aviz crushed the Castilians and their allies. This effectively killed off any realistic prospect of Castile conquering and absorbing Portugal. It confirmed Joao’s somewhat shaky right to call himself Joao I, king of Portugal. And it began a mighty age in Portuguese history.

The battle of Aljubarrota, 14th August 1385

The Treaty of Windsor, agreed after negotiations in England the following year, gave detailed and honorable form to the Anglo-Portuguese alliance. By now, grand plans were afoot. The king’s uncle, John of Gaunt, had been persuaded by the Portuguese to lay claim to the throne of Castile for himself. (His late elder brother, the Black Prince, had briefly considered doing this in the 1360s; Gaunt had also married a Castilian princess, Constance, in whose right he could claim the crown.) The Portuguese in their turn had agreed to supply ships - fast galleys with strong crews and cannon on board - to the English, who were worried about the prospect of full amphibious invasion by the French.

The treaty was negotiated in April, drawn up at the royal chancery in Westminster, then sealed and witnessed in the chapter house of St George’s Chapel in Windsor on May 9th 1386, in the presence of a grand delegation of dukes, earls, bishops, knights and clerks. Portuguese seals were added back at Westminster a week later, after which the text was sent to Coimbra in Portugal to be approved by Joao I. Finally, it was returned to England - and it can still be found in the National Archives today.

Prince Henry the Navigator, son of Joao I and Philippa of Lancaster

There were several important consequences to the Treaty of Windsor. Most immediately it committed the English to further involvement on the Iberian Peninsula. John of Gaunt took a large expedition there in the summer of 1386, and though he achieved very little militarily, he ensured that the Lancastrian dynasty would put down roots in both Portugal and what would become Spain.

We tend to think of the Lancastrians solely in terms of the Wars of the Roses, which pitted various factions of Gaunt’s bloodline against the house of York for control of England. But in 1387 Gaunt’s daughter Philippa married Joao I of Portugal, and the dynasty they created (the house of Aviz) laid the foundations for Portuguese maritime expansion during the Age of Discovery - effectively the creation of Portugal as a world imperial power.

Their offspring and descendants included Henry the Navigator, Manuel I and many others besides. It is worth remembering that while one of Gaunt’s grandsons, Henry V, was fiddling around in northern France winning the battle of Agincourt, another of them, Henry the Navigator, was laying the foundations for full-blown African and transatlantic empire.

However, Gaunt played both sides. In 1388, having failed to conquer Castile, he concluded a peace deal, under the terms of which he was paid a fortune in silver ingots and coin. To seal this deal he married another of his daughters, Catherine, to the young Castilian prince Henry. Two years later that boy became Henry III of Castile and Catherine was the queen. This brought to an end the Iberian phase of the Hundred Years War and it inserted the Lancastrian bloodline into yet another European royal house.

Yet it did not undermine the terms of the Treaty of Windsor with the Portuguese. That remained in force throughout the Middle Ages, and although effectively superseded in international relations (Portugal and the United Kingdom are today effectively allied by virtue of their membership of NATO) its spirit never been broken since.

• If you want to read more about the Treaty of Windsor in context, I recommend Divided Houses: The Hundred Years War III by Jonathan Sumption (Faber & Faber: 2009).

• ‘The Treaty of Windsor (1386) in a European Context’ by Malcolm Vale is also a very readable and short primer.