The War of the Castilian Succession (1475—1479) followed Henry IV of Castile's death without a legitimate successor.
Henry's half-sister Isabella claimed the throne, as did Portugal's Afonso V, who was married to Henry's reputed daughter Juana 'la Beltraneja' de Trastámara.
Since Isabella was married to Ferdinand, heir apparent to the Crown of Aragon, France intervened to support the Portuguese side, hoping to secure disputed territories in Italy and Roussillon.
While Juana's side enjoyed some initial success, the Courts of Madrigal-Segovia recognised Isabella's claim after the indecisive Battle of Toro (1476) and the French withdrew from the conflict.
While the war between Castile and Portugal continued for another two years, the focus shifted to the Atlantic and a struggle for maritime access to Guinea's slaves and gold.
After a decisive Portuguese victory at the Battle of Guinea (1478) sealed that issue, the Treaty of Alcáçovas recognized Isabella as ruler of Castile and granted Portugal hegemony in the Atlantic, apart from the Canary Islands, which remained in Castilian hands.