John I of Castile

Upon the death of King Ferdinand I of Portugal without a male heir, John I of Castile asserted his claim to the Portuguese throne through his wife Beatrice, daughter of Ferdinand I. This claim was contested by Portuguese nobles, leading to the 1383-1385 Portuguese interregnum.

John I of Castile led his army against Portuguese forces commanded by John of Aviz at Aljubarrota. The Castilian army was decisively defeated, securing Portuguese independence and ending Castilian claims to the Portuguese throne. The battle established the Aviz dynasty in Portugal.

Following the defeat at Aljubarrota, John I of Castile signed the Treaty of Windsor with England, formalizing an alliance between Castile and England. The treaty included provisions for mutual military support and trade agreements, strengthening Castile's position in the Hundred Years' War.