War of the Castilian Succession
When Castile's Henry IV died in December 1474 without a recognised will or a clear heir to the throne, his daughter Juana de Trastamara and half-sister Isabella claimed the throne. According to the earlier Treaty of the Bulls of Guisando, the throne was Isabella's, but she was not to marry without Henry's permission. When she secretly married Crown Prince Ferdinand of Aragon in 1469 without his approval, as far as Henry was concerned, the treaty was voided, and he reinstated Joanna as his successor. Both claimants had supporters among the Castilian nobility, so both were proclaimed queens by their supporters. With Isabella linked to one of the Iberian kingdoms, Joanna's supporters suggested she marry her uncle, a widower, who happened to be Portugal's Afonso V.
After rivalry between Aragon and France over control of Rousillon and hegemony in Northern Italy saw Louis XI take Joanna's side, the War of the Castilian Succession (1475 — 1479) became, effectively, Aragon versus Portugal and France over who should rule Castile. On the sidelines, France was also at war with England and Burgundy, and Navarre was distracted by an on-and-off civil war. Muslim Granada remained neutral even though Portugal tried to bring them in on Joanna's side.
After some initial successes, Joanna's alliance broke up after the Battle of Toro (1476), and when Ferdinand and Isabella declared a decisive victory after the indecisive battle of Peleagonzalo, the succession was settled.
Hostilities between Portugal and Castile's new rulers continued, with the focus switching to naval warfare in the Atlantic as Castilian merchants sought to break into the Portuguese monopoly in West Africa. The decisive Battle of Guinea ensured Portuguese control of those waters before the 1479 Treaty of Alcáçovas recognized Isabella and Ferdinand as sovereigns of Castile and Portugal hegemony in the Atlantic, effectively drawing a line across the Atlantic south of the Canary Islands. Joanna lost her right to the throne of Castile and remained in Portugal until her death.
