The world's oldest active military alliance was forged in 1386 and still holds today.

Ideas & Philosophy · Published 3 October 2026 Swipe more like this →

In short: Spanning over six centuries, the Anglo-Portuguese Alliance remains the longest-running bilateral agreement in history. From medieval crusades to the strategic airfields of the Second World War, this enduring pact has survived dynastic shifts, revolutions, and global conflicts, proving that even the most ancient treaties can adapt to modern geopolitical realities.

The Anglo-Portuguese Alliance traces its formal origins to the 1386 Treaty of Windsor, though its roots extend to the 1373 Anglo-Portuguese Treaty and even earlier military cooperation during the 1147 Siege of Lisbon. This pact of perpetual friendship was cemented by the marriage of Philippa of Lancaster to King John I of Portugal in 1387. Philippa, daughter of John of Gaunt, introduced Anglo-Norman traditions to the Portuguese court, fostering a generation of princes who propelled Portugal into its golden age of discovery.

The alliance has faced significant tests, most notably during the 60-year Iberian Union (1580–1640), when Portugal was subsumed by Spain. During this period, the two nations found themselves on opposite sides of the Anglo-Spanish War. However, the alliance was restored following the rise of the House of Braganza in 1640. By the 20th century, British officials questioned the treaty's relevance, but a 1927 review concluded that the alliance remained strategically vital, particularly for denying Atlantic island bases to potential adversaries.

During the Second World War, the treaty proved its worth once more. While Portugal maintained a policy of strict neutrality, the alliance provided the diplomatic framework for Britain to secure access to the Azores in 1943. This access allowed the United States to establish Lajes Field, a critical node that reduced transatlantic flight times from 70 to 40 hours and facilitated the movement of thousands of aircraft and wounded personnel. Today, the alliance continues to be reaffirmed through modern diplomatic instruments, such as the 2022 UK-Portugal Joint Declaration, ensuring its status as a cornerstone of international cooperation.

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