On June 13, 1373 the Anglo-Portuguese treaty was signed by King Edward III of England and King Ferdinand and Queen Eleanor of Portugal, establishing a treaty of “perpetual friendships, unions, [and] alliances” between the two countries. The treaty, which became the Anglo-Portuguese Alliance thirteen years later with the Treaty of Windsor, is still in effect and is thought to be the oldest treaty in the world still in force.
The two seafaring nations had been friendly for some time before the treaty came about. In the mid-12th century, English Crusaders on their way to the Holy Land stopped for a bit and helped Portuguese King Alfonso Henriques conquer Lisbon, and the two countries became allies as early as 1294. With the Treaty of Windsor, the countries promised to come to the aid of each other when necessary, and to seal the deal, John of Gaunt, son of Edward III of England, married his daughter, Philippa, to King John I of Portugal. She and her sons, who included Ferdinand the Saint Prince and Prince Henry the Navigator, left an indelible mark on Portuguese history.
The relationship between the two nations wasn’t always a good one. A dynastic crisis in Portugal in the late 16th century resulted in Spain taking over the country, which meant Portuguese foreign policy got tangled with Spain’s hostility to England. As a result, England and Portugal ended up on opposite sides during the Anglo-Spanish War (1585-1604) and the Dutch-Portuguese War. The alliance was reestablished after the English Restoration, by which time Portugal had thrown off the rule of the Spanish Habsburgs and installed their own king once more, John IV of Portugal, a member of the House of Braganza. There was another hiccup during the War of the Spanish Succession, which had Portugal originally side with France before reuniting with Britain after the Battle of Blenheim.
During the Seven Years’ War, Britain intervened when Spain invaded Portugal, and despite being vastly outnumbered, the two countries were able to force a stalemate. Not long after, during the Napoleonic Wars, Portugal refused to stop trading with its old ally, drawing the wrath of the French, who invaded. The British government urged the Portuguese royal family to flee to their colony of Brazil for safety, which they did, escorted by a British fleet.
In 1890, the British government delivered the British Ultimatum to Portugal, which forced the retreat of the Portuguese military from land between the Portuguese colonies of Mozambique and Angola. The British wanted that area to create a railway linking its own colonies throughout Africa. The Portuguese were stung by this betrayal, which fed Republican outrage against the king and government. Eventually, this outrage led to the assassination of King Carlos I and his eldest son in 1908 and the revolution that ended the Portuguese monarchy in 1910.
Relations were more cordial in the 20th century. During World War II, Portugal remained neutral but allowed Britain and her allies to establish bases on the Azores. These same facilities were offered to the Royal Navy during the 1982 Falklands War.