Isabella was the eldest child of
Ferdinand II of Aragon and
Isabella I of Castile. Born during the reign of her uncle,
Henry IV of Castile, the early years of her life were defined by the tension between him and her mother, as her uncle would not forgive her mother for marrying Ferdinand without his permission. Upon the death of Henry IV in 1474, Isabella's mother claimed the throne of Castile, and the young Isabella was swiftly sworn as the heiress presumptive to the throne. The early years of the reign of Isabella I were spent embroiled in a war of succession, as Henry IV had not specifically named a successor. A struggle ensued between Isabella I and her niece
Joanna, who was known as "la Beltraneja" due to the rumors that she was the illegitimate child of Henry IV's queen
Joan of Portugal and his favourite,
Beltrán de la Cueva, 1st Duke of Alburquerque.
Afonso V of Portugal, who was Henry IV's brother-in-law and young Joanna's uncle, intervened on Joanna's behalf and Ferdinand and Isabella were forced into a war with Portugal. During the war, young Isabella witnessed some of the chaos for herself. While her parents were fighting the Portuguese, the princess was left in
Segovia while the city was placed under the control of Andrés de Cabrera and his wife
Beatriz de Bobadilla. The city's residents, unhappy with this new administration, rose up and seized control of the city. The then-seven-year-old princess was trapped in a tower of the Alcázar for some time until her mother returned to Segovia and took control of the situation. The war ended in 1479 with the
Treaty of Alcáçovas. Among the terms were the provision that Princess Isabella would marry the grandson of Afonso V,
Afonso, who was five years younger than the princess. The treaty also provided that Ferdinand and Isabella would pay a large dowry for their daughter and that the princess would reside in Portugal as a guarantee that her parents would abide by the treaty terms. In 1480, Prince Afonso went to live in the town of
Moura with his maternal grandmother
Beatrice, Duchess of Viseu, and was joined in the early months of the following year by his future wife, the ten-year-old Isabella. She spent three years in Portugal before returning home. Isabella also spent a considerable part of her youth on campaign with her parents as they conquered the remaining Muslim states in southern Spain. For example, she accompanied her mother in accepting the surrender of the city of
Baza. ==Marriages==