The Treaty of Windsor was signed in 1386 between the Kingdom of England and the Kingdom of Portugal. This agreement established a strong alliance, promoting mutual support in military and trade matters. It marked a significant moment in the relationship between the two nations, fostering cooperation that would last for centuries. The treaty also included provisions for the marriage of the English royal family to the Portuguese royal family, further solidifying their bond. This alliance played a crucial role in the political landscape of Europe during the late Middle Ages, influencing future diplomatic relations between England and Portugal.