John I of Castile vs Margaret II of Avesnes

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.

Margaret II inherited the counties of Hainaut, Holland, and Zeeland after the death of her brother William II at the Battle of Warns. She became the last Avesnes ruler of these territories.

Margaret II fought a war against her son William I of Bavaria (Wittelsbach) for control of Holland and Zeeland. The conflict ended with the Treaty of 1354, where she ceded these territories to William.

Margaret II of Avesnes, Countess of Hainaut, ceded the counties of Holland and Zeeland to her son William I of Bavaria (Wittelsbach) after a period of conflict. This transfer ended Avesnes rule in Holland and Zeeland.