John I of Castile vs William II Rufus

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.

William was crowned king of England on September 26, 1087, after the death of his father William the Conqueror. He inherited the English throne while his brother Robert Curthose received Normandy.

William faced a rebellion led by Norman barons supporting his brother Robert Curthose. William suppressed the revolt by promising reforms and confiscating rebel lands.

William quarreled with Archbishop Anselm over church revenues and authority. Anselm went into exile in 1097, and William seized the revenues of the archbishopric.

William was killed by an arrow while hunting in the New Forest on August 2, 1100. The circumstances were suspicious, and his brother Henry I quickly seized the throne.