The Longest Unbroken Treaty in the World Is Between England and Portugal

For over 650 years, a formal bond of "perpetual friendship" has connected England and Portugal, making their alliance the most durable in recorded history. This pact was not born from simple goodwill, but from the complex power struggles of medieval Europe. The original Treaty of London was signed in 1373, a time when England was embroiled in the Hundred Years' War with France. England needed a foothold on the continent to challenge France's allies, particularly the powerful kingdom of Castile. Portugal, fearing absorption by its larger neighbor Castile, saw England as a formidable protector (Review). This mutual need formed the bedrock of an agreement promising military aid and commercial ties.

The alliance is far from a dusty historical artifact; it has been actively invoked throughout the centuries. During the Napoleonic Wars, Britain sent troops to defend Portugal from French invasion. In the 20th century, the treaty shaped major global conflicts. Although officially neutral for much of World War II, Portugal granted the Allies vital access to naval and air bases in the Azores, which proved critical in the Battle of the Atlantic. Even as recently as the 1982 Falklands War, Portugal supported the United Kingdom, demonstrating the remarkable longevity and practical relevance of this medieval promise.