The Anglo-Portuguese Treaty of 1373 was signed between King Edward III of England and King Ferdinand and Queen Eleanor of Portugal. It established a treaty of "perpetual friendships, unions [and] alliances" between the two seafaring nations. It is the oldest active treaty in the world.
It was reinforced throughout history, including in 1386, 1643, 1654, 1660, 1661, 1703, 1815 and by a secret declaration in 1899. It was recognized in the Treaties of Arbitration in the 20th century between Britain and Portugal in 1904 and 1914.
The treaty was temporarily void during the Iberian Union from 1580 to 1640, when Spain and Portugal were united. However, with Portugal's independence, the alliance returned & came to a new height during the Napoleonic wars when the British sent their best general, the Duke of Wellington, to sap Napoleon's armies in the Peninsular War.
It was activated again during the Second World War, whereupon the Portuguese remained neutral, in agreement with Britain, which did not want to bring the war into the Iberian Peninsula, until 1943, when it was fully reactivated by the National Government of Winston Churchill and Portugal. Britain, after three months' negotiations, was accorded aerodrome and nautical facilities in the Portuguese Azores to help combat the U-boat threat. The British also cited the treaty during the 1982 Falklands War.
Despite the fact that the treaty has technically been in effect for nearly 650 years does not mean that it has always been practically carried out. Widespread war has rarely been waged between the two, but that does not mean it has never been. In the Dutch-Portuguese War, the British (and to a lesser extent the French), seeking to curtail Habsburg power, openly aided the Dutch in their war on Portugal, which was by then a dependent vassal of Habsburg Spain.
An excerpt of the treaty is given below:
"In the first place we settle and covenant that there shall be from this day forward... true, faithful, constant, mutual and perpetual friendships, unions, alliances, and needs of sincere affection, and that as true and faithful friends we shall henceforth, reciprocally, be friends to friends and enemies to enemies, and shall assist, maintain, and uphold each other mutually, by sea and by land, against all men that may live and die."