What is the longest alliance between two nations still in force today?

It is 630 years since a treaty of alliance between King Richard II of England and King John I of Portugal was ratified at Windsor on May 9, 1386. As the longest continuing alliance in global history, The Treaty of Windsor of 1386 is significant because it effectively cemented and strengthened ties between the two kingdoms and helped convert a fledgling alliance into a more permanent legacy of history.

The Anglo-Portuguese alliance had in fact been born primarily out of converging strategic interests. An alliance between France and Spain had caused consternation at the English court. Through the alliance with Spain, which had one of the largest fleets in Western Europe, France could utilize Spanish sea power in its struggle against England. Therefore a closer union with Spain's western neighbor Portugal was a logical step for England to counter this threat.

The terms of the treaty included provisions for guaranteeing the mutual security of both nations and strengthening of commercial ties through trade. There were even clauses encouraging freedom of movement and settlement between the two countries, as subjects of either country had the right to dwell in the domains of the other. Winston Churchill in a speech in the House of Commons in October 1943 famously described the unique and ancient friendship between England and Portugal as an alliance “without parallel in world history”.