linguistic:translation, structural:reformatting
Coronation and war with Portugal

On 12 December 1474, news of King Henry IV's death in Madrid (which had happened on 11 December) reached Segovia. This prompted Isabella to take refuge within the walls of the Alcázar of Segovia, where she received the support of Andres de Cabrera and Segovia's council. The next day, Isabella was proclaimed Queen of Castile and León.

Isabella's reign got off to a rocky start. King Henry IV had named Isabella as his successor, so when she ascended to the throne in 1474, there were already several plots against her. Diego Pacheco, the Marquis of Villena, and his followers maintained that Joanna la Beltraneja, Henry's daughter, was the rightful queen. Shortly after the Marquis made his claim, the Archbishop of Toledo — his great-uncle and a longtime supporter of Isabella — left court to plot with him. They made plans to have Joanna marry her uncle King Afonso V of Portugal and invade Castile to claim the throne for themselves.

In May 1475, King Afonso and his army crossed into Spain and advanced to Plasencia. Here he married the young Joanna. A long and bloody war for the Castilian succession then took place. The war went back and forth for almost a year, until the Battle of Toro on 1 March 1476, in which both sides claimed and celebrated victory: the troops of King Afonso V were beaten by the Castilian centre-left commanded by the Duke of Alba and Cardinal Mendoza while the forces led by John of Portugal defeated the Castilian right wing and remained in possession of the battlefield.

But despite its uncertain outcome, the Battle of Toro represented a great political victory for the Catholic Monarchs, assuring them the throne since the supporters of Joanna la Beltraneja disbanded and the Portuguese army, without allies, left Castile.As summarized by the historian Justo L. González:

Both armies faced each other at the camps of Toro resulting in an indecisive battle. But while the Portuguese King reorganized his troops, Ferdinand sent news to all the cities of Castile and to several foreign kingdoms informing them about a huge victory where the Portuguese were crushed. Faced with these news, the party of "la Beltraneja" was dissolved and the Portuguese were forced to return to their kingdom.

With great political vision, Isabella took advantage of the moment and convoked courts at Madrigal-Segovia (April–October 1476) where her eldest child and daughter Isabella was first sworn as heiress to Castile's crown. That was equivalent to legitimizing Isabella's own throne.

In August of the same year, Isabella proved her abilities as a powerful ruler on her own. A rebellion broke out in Segovia, and Isabella rode out to suppress it, as her husband Ferdinand was off fighting at the time. Going against the advice of her male advisors, Isabella rode by herself into the city to negotiate with the rebels. She was successful and the rebellion was quickly brought to an end. Two years later, Isabella further secured her place as ruler with the birth of her son John, Prince of Asturias, on 30 June 1478. To many, the presence of a male heir legitimized her place as ruler.

Meanwhile, the Castilian and Portuguese fleets fought for hegemony in the Atlantic Ocean and for the wealth of Guinea (gold and slaves), where the decisive naval Battle of Guinea was fought.