The friendship between England and Portugal goes back to 1147 when English crusaders helped King Alfonso I capture Lisbon from the Muslims. This led to an English colony in Lisbon and to frequent contacts between the nobles of the two kingdoms.
In 1386 Richard II of England agreed the Treaty of Windsor with John I of Portugal. English troops had recently helped John drive out a Spanish invasion and it was decided to turn the friendship into a formal alliance. The key section of the treaty reads: “If, in time to come, one of the kings or his heir shall need the support of the other, the ally shall be bound to give aid and succour to the other, to the extent required by the danger to his ally’s realms, lands, domains, and subjects.”
In 1580 Portugal was overrun by Spain, but when Portugal regained independence in 1640 its first act was to reconfirm the Treaty of Windsor. The treaty has been invoked several times. Portugal joined Britain in the War of the Spanish Succession in 1704; in 1808 Britain sent troops to drive out a French invasion; in the First World War Portugal declared war on Germany; in the Second World War Portugal allowed Britain to use bases on the Azores – and repeated the favour in the 1982 Falklands War.
It has not all been plain sailing, however. In 1890 the two nations almost came to blows over what is now Zambia and in 1961 Britain refused to help Portugal when India invaded Goa.
Answered by Rupert Matthews, historian and author.