In 1943 Churchill invoked a treaty signed in 1373

Winston Churchill stood up in the House of Commons on 12 October 1943, in the middle of the Battle of the Atlantic, and opened like this: I have an announcement to make to the House arising out of the treaty signed between this country and Portugal in the year 1373 between His Majesty King Edward III and King Ferdinand and Queen Eleanor of Portugal.

Then he paused to let the date sink in. It took the members a second to understand that it was not a joke. Churchill was not asking for anything: he was announcing that Portugal had just granted him facilities — that was the word, not bases — in the Azores, the Portuguese islands in the middle of the Atlantic, to protect merchant shipping and to hunt German submarines in the stretch of ocean that aircraft could not reach from land. And the agreement rested on a document five hundred and seventy years old.

The document exists and is still in force. The alliance between England and Portugal began with that treaty of 1373, signed by Edward III on one side and Ferdinand I and Leonor Teles on the other, and it was signed again, stronger and more solemn, on 9 May 1386 at Windsor. What Churchill read out that day was article one of the 1373 treaty, which promises perpetual friendships, unions and alliances; the Windsor text repeats it even more solemnly, a perpetual league meant to last for ever. Neither of them sets an expiry date anywhere.

Behind it there was a battle. On 14 August 1385, at Aljubarrota, north of Lisbon, the Portuguese army of John I stopped the Castilians, and it did so with some two hundred English archers on the field, veterans of the Hundred Years War, the sort who drew the longbow, the weapon that forty years earlier had shredded the French cavalry in northern France. Portugal kept its crown. Eighteen months later, on 2 February 1387, John I married Philippa of Lancaster, daughter of John of Gaunt, in Oporto. One of their sons was Prince Henry the Navigator, who mounted the expeditions that took Portugal down the African coast.

Since then the alliance has woken up whenever it was needed. In 1899, with the Boer War under way, the two governments signed a secret declaration reaffirming it. In February 1916 Portugal seized the German ships moored in Lisbon because London asked it to, thirty-six of them in one go on the 23rd, and Germany declared war two weeks later, on 9 March. And in 1943 came the turn of the Azores: Portugal was officially neutral, António de Oliveira Salazar did not want a false step with Berlin, and the medieval treaty gave him the legal cover to say yes without ceasing to be neutral. The Allies moved into the Lajes base that same October, and from Terceira island the patrol aircraft finally reached the middle of the Atlantic, the corridor that until then no plane could cover from land.

Between the signing at Windsor and that speech, five hundred and fifty-seven years went by, and between the speech and today, another eighty-three. Six hundred and forty years without either side denouncing the treaty: it is the oldest alliance in force in the world.