Between which two countries is the oldest alliance in the world that is still in force?

The Anglo-Portuguese Alliance (or Aliança Luso-Britânica), ratified at the Treaty of Windsor in 1386, between England (succeeded by the United Kingdom) and Portugal is the oldest alliance in the world that is still in force – with the earliest treaty dating back to the Anglo-Portuguese Treaty of 1373.

This alliance, which goes back to the Middle Ages, has served both countries. It was very important throughout history, influencing the participation of the United Kingdom in the Iberian Peninsular War, the UK's major land contribution to the Napoleonic Wars and the establishment of an Anglo-American base in Portugal. Portugal aided England (and later the UK) in times of need, for example, in the First World War.

English aid to the House of Aviz (which ruled Portugal from 1385 to 1580) set the stage for Portuguese cooperation with England that would become a cornerstone of Portugal's foreign policy for more than five hundred years. However, English aid to Portugal went back much further to the 1147 Siege of Lisbon, when English and other northern European crusaders – en route to the Holy Land to participate in the Second Crusade – stopped and helped Portuguese King Afonso Henriques to conquer the city from the Moors. In May 1386, the Treaty of Windsor sealed the alliance - first started in 1294, renewed in the Anglo-Portuguese Treaty of 1373 and confirmed at the Battle of Aljubarrota (1385) - with a pact of perpetual friendship between the two countries.