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By Open Chronicle
On 13 June 1373, representatives of the kingdoms of England and Portugal signed the Anglo-Portuguese Treaty of Perpetual Alliance in London, creating what is widely regarded as the oldest diplomatic and military alliance between two sovereign states that remains in force today. The agreement, concluded during the reigns of King Edward III of England and King Ferdinand I of Portugal, laid the foundation for more than six centuries of political, military, and economic cooperation.
The treaty emerged during a period of intense instability across Western Europe. England was deeply engaged in the Hundred Years’ War against France, while Portugal sought to preserve its independence amid growing pressure from the Kingdom of Castile. By formalising a permanent alliance, both monarchies sought mutual security against common rivals and pledged lasting friendship and assistance.
The original agreement committed the two kingdoms to “perpetual friendships, unions, alliances and confederations,” promising that each would aid and support the other in times of need. Although medieval treaties were often short-lived or overturned by changing political circumstances, the Anglo-Portuguese accord proved remarkably durable.
The alliance was strengthened in 1386 through the Treaty of Windsor, which reaffirmed the commitments of 1373 and further deepened diplomatic ties. The following year, the marriage of King John I of Portugal and Philippa of Lancaster, daughter of John of Gaunt, cemented the relationship through dynastic union. Their descendants, known as the “Illustrious Generation,” played a significant role in launching Portugal’s Age of Discoveries.
Over the centuries, the alliance influenced major events in European and global history. During the Napoleonic Wars, British forces under the command of the Duke of Wellington fought alongside Portuguese troops against the French invasions of the Iberian Peninsula. In the nineteenth century, Britain remained an important trading partner and strategic ally as Portugal maintained its overseas empire.
The treaty also retained symbolic and practical relevance in the twentieth century. During the Second World War, Portugal remained officially neutral. Still, the longstanding alliance facilitated negotiations that allowed the United Kingdom to use military bases in the Azores, a decision that proved strategically valuable for Allied operations in the Atlantic.
Today, the Anglo-Portuguese Alliance continues as a cornerstone of the close relationship between the United Kingdom and Portugal. While both countries are now linked through wider international institutions and partnerships, including NATO, the 1373 treaty remains a unique example of enduring diplomacy, reflecting centuries of mutual trust, cooperation, and shared strategic interests.
More than 650 years after it was first signed in London, the Anglo-Portuguese Treaty of Perpetual Alliance stands not only as a remarkable legal and historical document, but also as a symbol of one of the longest and most resilient friendships in international relations.
Staff Writers at Open Chronicle produce in-depth, field-informed reporting on defense, diplomacy, cultural transformation, and global affairs. Known for clarity, accuracy, and analytical depth, they connect breaking developments to broader historical and strategic contexts. In addition to frontline journalism, Staff Writers also contribute to the Open Chronicle Encyclopedia, crafting authoritative entries that preserve critical knowledge and enrich public understanding.