9th May 1386: Treaty of Windsor, the world's longest-lasting treaty, signed by Portugal and England

The Treaty of Windsor was negotiated by King John I of Portugal and John of Gaunt, the Duke of Lancaster, on behalf of the English king Richard II. The Portuguese king had secured his rule by defeating the Crown of Castile at the Battle of Aljubarrota the previous year, where he had been assisted by English troops that included a number of skilled longbowmen. The treaty solidified the existing diplomatic relationship between England and Portugal with the establishment of a mutual defence pact. This provision helped to deter potential aggression from Castile and provided both England and Portugal with a sense of security. Additionally, the Treaty of Windsor paved the way for increased trade and economic cooperation between England and Portugal. It encouraged the exchange of goods and services, as well as the development of commercial ties, which benefited both countries. The treaty had significant cultural and diplomatic implications. It facilitated exchanges of ambassadors, diplomats, and scholars between England and Portugal, fostering cultural exchange and mutual understanding. Furthermore, it helped to strengthen the position of both countries on the international stage, as they were able to present a united front against external threats. This was secured in 1387 with the marriage of John I of Portugal to Philippa of Lancaster, the daughter of the John of Gaunt. The Treaty of Windsor remains in force, making it the longest-lasting diplomatic treaty in recorded history, and even served as the basis for Portuguese assistance to Britain during the Second World War.