England spearheaded the Anglo-Spanish War (1585–1604) on the side of the deposed Portuguese royal house.
The alliance has served both countries throughout their respective military histories, 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.
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.
The most important part of the treaty stated that: It is cordially agreed that if, in time to come, one of the kings or his heir shall need the support of the other, or his help, and in order to get such assistance applies to his ally in lawful manner, the ally shall be bound to give aid and succour to the other, so far as he is able (without any deceit, fraud, or pretence) to the extent required by the danger to his ally’s realms, lands, domains, and subjects; and he shall be firmly bound by these present alliances to do this.
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.
The alliance was reconfirmed after the Portuguese Restoration War and the English Restoration.
The alliance was reconfirmed after the breakup of the Iberian Union, primarily due to both countries' respective rivalries with Spain, the Netherlands, and France, both in Europe and overseas.
Portugal aided England (and later the UK) in times of need, for example, in the First World War.
Today, Portugal and the United Kingdom are both part of NATO, a larger intergovernmental military alliance between several North American and European states that accounts for over 70% of total global military spending.