Henry IV of Castile vs Sancho II of Castile

Henry IV of Castile signed the Treaty of the Bulls of Guisando, recognizing his half-sister Isabella as his legitimate heir instead of his daughter Joanna. This agreement was intended to resolve the succession dispute, but Henry later repudiated it, leading to renewed conflict.

After Henry IV's death, a civil war erupted between supporters of his daughter Joanna la Beltraneja and those of his half-sister Isabella. The war involved Portuguese and Aragonese intervention and ended with Isabella's victory, securing her throne and leading to the union of Castile and Aragon.

During the War of the Castilian Succession, Henry IV's forces lost control of the Canary Islands to Portuguese-backed rebels. The islands were later recovered by Isabella I, but the conflict highlighted the weakness of Henry's reign and the erosion of royal authority.

Sancho II conquered the Kingdom of Leon from his brother Alfonso VI, following the division of their father Ferdinand I's realm. He defeated Alfonso at the Battle of Llantada and forced him into exile in Toledo. This conquest united Leon and Castile under Sancho's rule.

Sancho II conquered the Kingdom of Galicia from his brother Garcia II, who was captured and imprisoned. This conquest completed Sancho's unification of the realms of his father, making him the most powerful Christian ruler in Iberia.

Sancho II besieged the city of Zamora, held by his sister Urraca, who had rebelled against him. During the siege, Sancho was assassinated by a Zamoran noble named Vellido Dolfos. His death ended his unification efforts and allowed his brother Alfonso VI to reclaim the throne.