Succession to Crown of Castile
Joanna la Beltraneja, born in 1462, the first and only daughter of King Henry IV of Castile, was of Asturias. A rumour spread that Princess Joanna was not actually the daughter of King Henry but rather of Beltrán de la Cueva, the alleged lover of Queen Joan of Portugal. Joanna was thus nicknamed "la Beltraneja", as a mocking reference to her assumed father. Pressure from members of the nobility forced the King to strip her of the title and name his half-brother Alfonso as heir, in 1464.
In 1465, a group of nobility assembled in Ávila and overthrew King Henry, replacing him with Alfonso. That led to a war that ended in 1468 with the death of the 14-year-old Alfonso.
Henry IV regained the throne, but the title of heir became disputed between Joanna, his daughter, and Isabella, his half-sister. That was resolved via the Treaty of the Bulls of Guisando, which gave Isabella succession rights but restricted her marriage options. Isabella secretly married Ferdinand in 1469 at the age of 17, ignoring Henry IV's wishes.
Gradually, the couple gained a larger number of supporters and obtained a papal bull sanctioning their marriage from Pope Sixtus IV in 1472 and gained the support of the powerful Mendoza family in 1473.
When Henry IV died in December 1474, both candidates for the throne were proclaimed Queen of Castile by their respective supporters. Aware of their position of weakness against Isabella's supporters, Joanna's supporters proposed for the 43-year-old King Afonso V of Portugal, a widower for some 20 years, to marry Joanna, his niece, and assume the throne of Castile with her.
The Kingdom of France and the Crown of Aragon maintained a long-held rivalry for the control of Roussillon and, more recently, for hegemony in Italy. In June 1474, French troops invaded Roussillon and the Aragonese were forced to retreat. On the possibility that the heir to the throne of Aragon would also become King of Castile, Louis XI of France officially positioned himself on the side of Joanna and Afonso in September 1475.
France was simultaneously at war with the Duchy of Burgundy. That made Burgundy into theoretical allies of Isabella's supporters, but in practice, it continued its war against France without coordinating their actions with the Isabella alliance.
The Kingdom of England was also briefly at war with France with the disembarkation of King Edward IV in Calais in June 1475, but by a quick diplomatic response, Louis negotiated peace with Edward and signed the Treaty of Picquigny in August. Edward IV accepted a truce of nine years, in exchange for significant economic compensation, and returned to England.
The Kingdom of Navarre was experiencing an intermittent civil war, and the Muslim Kingdom of Granada remained neutral despite Portuguese efforts to draw it into the war.
Rivalry between Castile and Portugal in the Atlantic
Modern reconstruction of a Portuguese caravel
Throughout the 15th century, merchants, explorers and fishermen of Portugal and Castile had been penetrating further into the Atlantic Ocean. The possession of the Canary Islands was a point of contention between the two Crowns. Later on, the control of commerce with the territories of Guinea and Elmina, rich in gold and slaves, grew to a dispute of even greater importance.
During the first half of the century, Castile staged the conquest of a few of the Canary Islands (Lanzarote, Fuerteventura, Hierro, and La Gomera) by feudal pacts, first with Norman knights and later with Castilian nobles. Portugal opposed Castilian authority on the islands and continued the exploration of Guinea, with significant commercial benefits.
Beginning in 1452, Pope Nicholas V and his successor, Callixtus III, modified the previous policy of the neutrality of the Holy See and issued a series of bulls favourable to Portugal. They gave Portugal commercial control and ample religious authority over all of Guinea, and in areas "further beyond". The Holy See did not arbitrate the question of the Canaries, whose conquest had been left relatively suspended. The King of Portugal adopted a freer commercial policy that allowed foreign subjects to trade on the African coasts, in exchange for taxes.
In August 1475, after the start of the war, Isabella claimed that parts of Africa and Guinea belonged to Castile by right and incited Castilian merchants to sail to them. That initiated a naval war in the Atlantic.