After the death of King Henry IV of Castile in late 1474, the supporters of his reputed daughter, Joanna 'la Beltraneja', waged a war for the succession to the Crown of Castile against the supporters of his half-sister, Isabella, who prevailed by 1479. The war had a marked international character, as Isabella was married to Ferdinand, heir apparent to the Crown of Aragon, while Joanna married her uncle King Afonso V of Portugal at the suggestion of her supporters. France intervened in support of Portugal, as they were rivals with Aragon for territory in Italy and Roussillon.

Despite a few initial successes by the supporters of Joanna, a lack of military aggressiveness by Afonso V and the stalemateAs noted by Spanish scholars Luis Suárez Fernández, Juan de Mata Carriazo and by Manuel F. Álvarez: "Not a military victory, but a political victory, the battle of Toro is in itself, a decisive event, because it solves the civil war in favour of the Catholic Monarchs, leaving as a relic, a border clash between the two countries ..." in La España de los Reyes Católicos (1474-1516), Espasa-Calpe, 1969, p.163. in the Battle of Toro (1476) led to the disintegration of Joanna's alliance and the recognition of Isabella in the Courts of Madrigal-Segovia (April–October 1476): "In 1476, immediately after the indecisive battle of Peleagonzalo [near Toro], Ferdinand and Isabella hailed the result as a great victory and called Courts at Madrigal. The newly gained prestige was used to win municipal support from their allies ..." (Marvin Lunenfeld).Lunenfeld, Marvin. The council of the Santa Hermandad: a study of the pacification forces of Ferdinand and Isabella University of Miami Press, 1970, p. 27.

The war between Castile and Portugal alone continued. This included naval warfare in the Atlantic, which became more important: a struggle for maritime access to the wealth of Guinea (gold and slaves). In 1478, the Portuguese navy defeated the Castilians in the decisive Battle of Guinea.Historian Malyn Newitt: "However, in 1478 the Portuguese surprised thirty-five Castilian ships returning from Mina [Guinea] and seized them and all their gold. Another ... Castilian voyage to Mina, that of Eustache de la Fosse, was intercepted ... in 1480. ... All things considered, it is not surprising that the Portuguese emerged victorious from this first maritime colonial war. They were far better organised than the Castilians, were able to raise money for the preparation and supply of their fleets, and had clear central direction from ... [Prince] John." In A history of Portuguese overseas expansion, 1400-1668, Routledge, New York, 2005, pp.37,38.Bailey W. Diffie and George D. Winius. "In a war in which the Castilians were victorious on land and the Portuguese at sea, ..." in Foundations of the Portuguese empire 1415-1580, volume I, University of Minnesota Press, 1985, p.152.Battle of Guinea: Alonso de Palencia, Década IV, Book XXXIII, Chapter V ("Disaster among those sent to the mines of gold [Guinea]. Charges against the King..."), p.91-94.

The war concluded in 1479 with the Treaty of Alcáçovas, which recognized Isabella and Ferdinand as sovereigns of Castile and granted Portugal hegemony in the Atlantic, with the exception of the Canary Islands. Joanna lost her right to the throne of Castile and remained in Portugal until her death.

This conflict has also been called the Second Castilian Civil War, but this name may lead to confusion with the other civil wars that involved Castile in the 14th and 15th centuries. Some authors refer to it as the War of Portugal; however, this name clearly represents a Castilian point of view and implicitly denies Joanna's claim. At other times the term Peninsular War has been used, but it is easily confused with the Peninsular War of 1808–1814, part of the Napoleonic Wars. Some authors prefer the neutral expression War of 1475–1479.

Background Succession to Crown of Castile

Joanna, born in 1462, was the only child born to King Henry IV of Castile and was called Princess of Asturias as heir presumptive to the throne. A rumour spread that she was not actually the daughter of King Henry but rather of Beltrán de la Cueva, the alleged lover of Henry's wife, 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 presumptive in 1465.Gerli, p. 305

In June 1465, a group of nobility assembled in Ávila, deposed King Henry in effigy and declared Alfonso the king of Castile. In 1467, the rebels were defeated by Henry at Olmeda but the death of Alfonso in 1468 aroused suspicions and increased tensions among the nobles who now favored the accession of Henry's half-sister, Isabel of Castile.Gerli, pp. 305-306

The resulting dispute between supporters of Joanna and Isabella was resolved with the Treaty of the Bulls of Guisando, which gave Isabella succession rights but restricted her marriage options. Nevertheless, Isabella secretly married Ferdinand in 1469 at the age of 17, ignoring Henry IV's wishes.Gerli, pp. 305-306

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.

International alliances

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

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 enslaved people, 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.

Conflict Combatants in 1475 In favour of Joanna: Portugal France Some of the high Castilian nobility and descendants of Portuguese families who settled in Castile after 1385; the Archbishop of Toledo (Alfonso Carrillo de Acuña); the Estúñiga family, with land bordering Portugal and Navarrese ancestors; the Marquis of Villena (Diego López Pacheco); the Marquis of Cádiz (Rodrigo Ponce de León);According to Ciudad Ruiz, "Rodrigo Ponce de León was the principal member of the opposition to the kings in Andalucia along with Alfonso de Aguilar" but did not want to rebel but he maintained "his personal war against the Duke of Medina Sidonia." and the grandmaster of the Order of Calatrava (Rodrigo Téllez Girón).

In favour of Isabella: The Crown of Aragon The rest of the Castilian nobility: the powerful Mendoza family; the Manrique de Lara family; the Duke of Medina Sidonia (Enrique Pérez de Guzmán); Beltrán de la Cueva; the Order of Santiago and the Order of Calatrava, except its grandmaster. "...at the heat of the civil war for the succession of the Castilian throne, the commander [Spanish: comendador mayor], the treasurer, and other knights of the Order take the side of Queen Isabella against their grandmaster, supporter at that time of doña Juana."

The Duchy of Burgundy and the Kingdom of England were at war with France in 1475, but they did not coordinate their actions with the supporters of Isabella and so are rarely considered part of the Isabella alliance.

Fight for throne (May 1475 – September 1476) Afonso V enters Castile

A Portuguese army entered the territory of the Crown of Castile under the command of Afonso V on May 10, 1475, and advanced to Plasencia, where Joanna was expecting him. Joanna and Afonso were proclaimed sovereigns of Castile on May 25 and were married; the required Papal dispensation (Joanna was Afonso's niece) arrived a few months later. From Plasencia, they marched to Arévalo, with the intention of heading towards Burgos. There, Afonso hoped to be able to unite with any troops sent by his ally, Louis XI of France.

The castle of Burgos and the cities of Plasencia and Arévalo were controlled by the Estúñiga family, supporters of Joanna. The city of Burgos, controlled by the Fernandez de Velasco family, backed Isabella.

Afonso found fewer supporters in Castile than he expected and changed his plans, preferring to instead consolidate his control in the area closest to Portugal, in particular Toro, a city that received him favourably even though the garrison of the castle proclaimed itself loyal to Isabella. Zamora and other Leonese villages of the lower Douro also accepted him.

In La Mancha, Rodrigo Tellez-Giron, the Master of the Order of Calatrava, supporter of Joanna, conquered Ciudad Real. Rodrigo Manrique, treasurer of that same Order and the Master of the Order of Santiago, reconquered the city for Isabella.(Ruiz 2000)

Ferdinand concentrated an army in Tordesillas, and on July 15, he ordered it to march to seek an encounter with Afonso. Four days later, they arrived at Toro, where the King of Portugal avoided direct combat. Ferdinand, lacking the necessary resources for a prolonged siege, was forced to return to Tordesillas and disband his army. The castle of Toro surrendered to Afonso V, who returned to Arévalo to wait for the expected French intervention.