The Full Story
England and Portugal. The oldest alliance in the world. Signed in 1386 by the Treaty of Windsor, it is the longest-standing diplomatic alliance in modern history. In more than 6 centuries the two nations have never gone to war with each other, though the alliance lapsed while Portugal was under the Spanish Crown between 1580 and 1640.
Then Britain abolished the slave trade in 1807. And spent the next sixty years pressuring its oldest friend to do the same. Portugal was the largest single slave trader in the Atlantic. It carried more enslaved Africans to the Americas, especially to Brazil, than any other European power. Treaty after treaty was signed between London and Lisbon. Ship after ship was intercepted on the open ocean by the West Africa Squadron, often in violation of what Portugal considered its sovereign waters. The alliance nearly did not survive it.
Portugal formally abolished the slave trade in 1836. Slavery in Portugal's African colonies was not abolished until 1869. Sixty years of British pressure. The alliance held. It still holds today, invoked whenever Britain and Portugal have needed to act together, including during the Second World War when it allowed British forces to use the Azores.
Why This Matters
British diplomacy spent the nineteenth century doing something unusual: using its position as a great power to force other states, including its closest allies, to give up slavery. The cost was significant. The Portuguese alliance came close to collapse. It is one of the few examples in modern history of a country risking its oldest friendship over a moral principle, and still being able to repair it afterwards. Both sides understood, eventually, that the principle was worth it.