The allure of the alliance shows no sign of abating. In recent years we have NATO’s collective support for Ukraine in response to Russia’s invasion, the ‘no limits’ strategic partnership between Russia and China, and Aukus, the trilateral security pact recently defined as an ‘alliance at sea’ between Australia, the United Kingdom and the United States. Alliances are a common way of achieving peaceful relations, as well as a means of creating deterrence. But what makes them last, and for how long?

16 June 2023 will mark the 650th anniversary of the conclusion of the Treaty of London, one of the four principal treaties that mark the formation of the Anglo-Portuguese alliance – the oldest in the world that remains in force. For more than six centuries it has survived challenging historical events and emergencies, including two world wars, the rise and fall of empires, revolution and decolonisation, and European integration as well as globalisation. In 2022, a new joint declaration was agreed, committing the UK and Portugal to ‘defend freedom and democracy, free trade, human rights and the rule of law’. The origin of this oldest of alliances had similar, if less grand, aims and explains something of its longevity.

Ferdinand and John

The Anglo-Portuguese alliance was forged in the context of the Hundred Years War (1337-1453), which, although a conflict between England and France, involved most of Europe. Various rulers and political entities weighed in, lending their support to either the English or the French side. The expansion of the war into Iberia was facilitated by conflicts over the kingdom of Castile between King Peter and his half-brother Henry Trastámara. Henry killed Peter at the Battle of Montiel on 14 March 1369 and became king in his own right, but his claim to the throne of Castile was challenged by two others: Ferdinand I of Portugal, who had a claim as the legitimate great-grandson of Sancho IV of Castile, and John of Gaunt, duke of Lancaster and third son of Edward III of England. John had married Peter’s daughter, Constance of Castile, in 1371. War between the various claimants seemed inevitable. It was in this context that an embassy from John of Gaunt arrived in Portugal to negotiate the Treaty of Tagilde.

The treaty was concluded in the Church of São Salvador in Tagilde in July 1372. It set out that the king of Portugal and the duke of Lancaster were true friends and military allies. This was an attempt by John of Gaunt not only to have his own claim to the Castilian throne recognised by a foreign power, but also to entice the Portuguese king to join the fight against Henry Trastámara. For Ferdinand, the treaty provided an opportunity to assert the independence of the Portuguese kingdom against interference from powerful Castile.

The importance of this mutual recognition is evident in the text of the treaty, which records that it applied to Ferdinand I and John of Gaunt and the people and territories they respectively ruled over as kings of Portugal and Castile – but, in the case of John of Gaunt, also included that which he ruled over as duke of Lancaster. In early 1372, John had received permission from his father, the English king Edward, to incorporate the royal arms of Castile and Leon into his own. By doing so, he portrayed himself as both a king in his own right as well as duke of Lancaster. The Treaty of Tagilde did not amount to a full alliance between an English and a Portuguese king, but it was a contract with one of the English king’s most powerful subjects, in a geographical region where the English king was keen to extend his influence, in order to put pressure on his French rival and win the Hundred Years War. Written in the original Castilian language to reflect John’s claim as king, the Treaty of Tagilde was the first formal expression of an Anglo-Portuguese alliance. It is still preserved among the records of the Duchy of Lancaster – a large portfolio of property now belonging to Charles III.

A tale of treaties

The practical aim of the Treaty of Tagilde was a military campaign against John of Gaunt’s rival for the Castilian throne, Henry Trastámara, with the English attacking Castile and Aragon from the north, and Ferdinand’s Portuguese troops attacking from the west. The plan never came to fruition. In the wake of the treaty, Henry Trastámara invaded Portugal, besieging and plundering Lisbon in February and March 1373. A peace treaty between Castile and Portugal was hastily concluded in Santarém on 24 March 1373, after which Henry Trastámara lifted the siege of Lisbon and abandoned Portugal. Ferdinand I was forced to renounce his new English ally – John of Gaunt – and to join forces with Castile and France. Despite this, within three months Ferdinand was celebrating the conclusion of an alliance with Edward III of England, negotiated by the same ambassador, João Fernandes Andeiro, who had been involved in negotiating the Treaty of Tagilde and who had seemingly been expelled from Portugal in accordance with the Treaty of Santarém.

Some historians have referred to the duplicity of the Portuguese king in concluding the Treaty of Santarém, abandoning his English ally John of Gaunt, but negotiating a new alliance with his father, Edward III, almost immediately. Yet, Ferdinand was able to do this without ever breaching the obligations in Santarém because that treaty specifically named John of Gaunt as the main enemy and contracting party of Tagilde, rather than the king of England. Tagilde had seemingly been intended as a way to test the waters politically and militarily. In particular, it allowed both the Portuguese and the English to understand better what Henry Trastámara might do if provoked, and to act once this was clear.

The Treaty of Tagilde was effectively a constitutional extension of the power of the English king, Edward III, through his son John of Gaunt as claimant to the throne of Castile. Such limited treaties tend to imply a ruler’s consent or involvement in some way, and rulers usually reserved the right to extend, limit or exclude such treaties from any other treaty they themselves concluded. Once the new Castilian king’s response was clear – a swift, brutal campaign against his Portuguese neighbour – Edward III certainly exercised this right by concluding the first full alliance between an English and a Portuguese king.

This new treaty was ratified in London in June 1373 and was explicitly presented as an expansion of the alliance that had been ‘mutually agreed upon, contracted and written down’ between Ferdinand I, his wife Leonor and John of Gaunt. The first clause of the Treaty of London states that the respective kings and their successors, realms and subjects should ‘faithfully obey, true, faithful, constant, mutual, and perpetual friendships, unions, alliances, and leagues of sincere affection’; and that ‘they shall henceforth reciprocally be friends to friends and enemies to enemies, and shall assist, maintain, and uphold each other mutually, by sea and by land, against all men that may live or die’. The wording of the alliance, formalised in this treaty, has subsequently been confirmed on several occasions: in 1643, 1703, 1810 and 1914.

It's personal

There is another more intriguing context to the treaties of Tagilde and London revolving around the marriage diplomacy of the Portuguese king, Ferdinand. Following the death of King Peter of Castile in 1369, Ferdinand tried to press his own claim to the Castilian throne and embarked on two unsuccessful campaigns against Henry Trastámara. The campaigns resulted in the Treaty of Alcoutim of March 1371, in which Ferdinand renounced his claim to the throne of Castile and promised to marry the infanta Leonor of Castile, Henry’s daughter. Despite his promises, Ferdinand had, in fact, already developed an affection for Leonor Teles, the wife of one of his own courtiers. Swiftly procuring the dissolution of Leonor’s marriage, Ferdinand reneged on his promises to Henry and his daughter and married Leonor Teles in May 1372. His alliance with John of Gaunt followed shortly after.

The Portuguese chronicler Fernão Lopes, writing around half a century after these events, presents Leonor as a treacherous figure, who, aside from abandoning her children from her first marriage to become queen of Portugal, also later engaged in an affair with João Fernandes Andeiro, Ferdinand’s ambassador to England in 1371 and the negotiator of the treaties of Tagilde and London. The exact nature of the relationship between the Portuguese queen and João Fernandes Andeiro is unclear, but it is certain that he – a Galician in origin – had assisted Ferdinand on one of his military campaigns against Henry Trastámara and he was expelled from the kingdom of Castile as a result. He was subsequently also expelled from Portugal as one of the conditions of peace with the Castilian king in the Treaty of Santarém in 1173. It has further been suggested that it was João who pressured Ferdinand into concluding the alliance with Edward III of England, just three months after Santarém.

Whatever the truth of this matter, the chronicler gave both João and Leonor starring roles in the last years of Ferdinand’s reign and in the civil war that followed. In 1383, João negotiated a marriage between Ferdinand and Leonor’s daughter, Beatrice, and the new Castilian king, John I, before being assassinated by a party opposed to the union and its implications for Portugal’s independence from Castile. Upon her husband’s death in 1383, Leonor assumed the regency of Portugal on her daughter’s behalf, then resigned her powers in favour of her new son-in-law before opposing him, and was ultimately banished to a convent in Castile. These intrigues resulted in a short civil war, which was settled at the Battle of Aljubarrota in 1385. Here, John I of Portugal – Ferdinand’s half-brother – emerged victorious, defeating the army of John I of Castile, securing the Portuguese throne as well as the independence of the kingdom. He did so with English support and his victory was followed with formal recognition for the new king through the renewal of the Anglo-Portuguese alliance in the Treaty of Windsor of 1386. 

Free trade, safe movement

The importance of the treaties of Tagilde and London not only provided a legal foundation for the alliance to provide military aid and friendship; they also bestowed international recognition of legitimacy and authority upon those who entered into them. The treaties were important affirmations of Portugal as an independent entity from Castile (and, later, Spain), and each time the alliance was renewed, this was again confirmed. This is one reason why the alliance has lasted throughout centuries of conflict.

The Treaty of Windsor of 1386 added a new aspect to the Anglo-Portuguese relationship, which, although not expressed in the treaties of Tagilde and London, nonetheless must have been part of their rationale: namely, shared economic motives. A commercial treaty had been concluded in 1353 between Edward III and ‘the good men’ (les bones Gentz) – merchants and mariners of the towns, cities and ports of Portugal. This treaty built on long-standing commercial practices recorded as far back as the 12th century, when John of England had referred to the king of Portugal as his ‘dearest brother and friend’ and granted letters to Portuguese merchants entitling them to free and safe passage, access to local markets and redress in case of loss or damage to their goods or person. In 1226 and 1227, John’s son, Henry III, granted similar rights to up to 100 merchants and towns such as Oporto and Coimbra, and over the course of the 13th and early 14th centuries, new grants were issued. The right to free and safe movement of people and goods, and to redress for loss or damage to property or person, were essential ingredients for the alliance.

Keep it vague

The Anglo-Portuguese alliance can be said to have originated with four different treaties. The first was a commercial treaty between the English king, Edward III, and Portuguese merchants dating to 1353, which formalised long-standing customary practices. The Treaty of Tagilde followed in 1372, concluded between King Ferdinand I of Portugal and John of Gaunt, a claimant to the kingdom of Castile but also a son of the English king. The Treaty of London of 1373 between Edward III of England and Ferdinand I of Portugal formalised the Treaty of Tagilde and set out the political aim of being allies in perpetuity. Finally, in 1386 the Treaty of Windsor combined both the commercial and political aspects of the alliance into a single treaty concluded between Richard II of England and John I of Portugal.

These four treaties highlight that the Anglo-Portuguese alliance was built on, first, free trade, or at least privileged access to markets and free passage of people and goods; second, international recognition, an important diplomatic tool of authority and legitimacy in times of crisis; and third, promises of aid and to defend each other against enemies. These assurances are usefully vague: they allow each party to conclude other alliances with more defined obligations, without encroaching on the first agreement. From its earliest incarnation, the Anglo-Portuguese alliance had an inherent flexibility, which could be amended to suit either party’s circumstances at the point of renewal. This has contributed significantly to its survival. On the 650th anniversary of the Treaty of London, we are reminded that alliance-building is the most formidable diplomatic tool in a political entity’s arsenal – now, just as in the 14th century.

Jenny Benham is Reader in Medieval History at Cardiff University and author of International Law in Europe, 700-1200 (Manchester University Press, 2022).