King Charles III today attended a Service of Thanksgiving alongside the Portuguese president Marcelo Rebelo de Sousa to mark the 650th Anniversary of the Anglo-Portuguese Alliance at St James' Palace.
The Anglo-Portuguese Alliance is the oldest military alliance that is still in force by political bilateral agreement. It was established by the Treaty of Windsor in 1386, between the Kingdom of England (since succeeded by the United Kingdom) and the Kingdom of Portugal (now the Portuguese Republic), though the countries were previously allied via the Anglo-Portuguese Treaty of 1373.
That means that, today, Britain and Portugal are their respective oldest allies.
Britain - then the most powerful country in the world - entered the Napoleonic Wars to guard little Portugal against French invasion and Portugal provided Britain with help during World War I. The two countries aided each other during World War II and during the Falklands War Portugal allowed the Royal Navy to use its facilities in the Azores.