Marriage portrait of Isabella and Ferdinand, who married in 1469
At the time of their marriage on October 19, 1469, Isabella was eighteen years old and the heiress presumptive to the Crown of Castile, while Ferdinand was seventeen and heir apparent to the Crown of Aragon. They met for the first time in Valladolid in 1469 and married within a week. From the start, they had a close relationship and worked well together. Both knew that the crown of Castile was "the prize, and that they were both jointly gambling for it". However, it was a step toward the unification of the lands on the Iberian peninsula, which would eventually become Spain.
They were second cousins, so in order to marry they needed a papal dispensation that Pope Paul II, an Italian pope opposed to Aragon's influence on the Mediterranean and to the rise of monarchies strong enough to challenge the Pope, refused to grant, so they falsified a papal bull of their own. Even though the bull is known to be false, it is uncertain who was the material author of the falsification. Some experts point at Carrillo de Acuña, Archbishop of Toledo, and others point at Antonio Veneris. Pope Paul II would remain a bitter enemy of Spain and the monarch for all his life, and is attributed the quote, "May all Spaniards be cursed by God, schismatics and heretics, the seed of Jews and Moors."
Isabella's claims to it were not secure, since her marriage to Ferdinand enraged her half-brother Henry IV of Castile and he withdrew his support for her being his heiress presumptive that had been codified in the Treaty of the Bulls of Guisando. Henry instead recognized Joanna of Castile, born during his marriage to Joanna of Portugal, but whose paternity was in doubt, since Henry was rumored to be impotent. When Henry died in 1474, Isabella asserted her claim to the throne, which was contested by thirteen-year-old Joanna. Joanna sought aid of her husband (who was also her uncle), Afonso V of Portugal, to claim the throne. This dispute between rival claimants led to the War of 1475–1479. Isabella called on the aid of Aragon, with her husband, the heir apparent, and his father, Juan II of Aragon providing it. Although Aragon provided support for Isabella's cause, Isabella's supporters had extracted concessions, Isabella was acknowledged as the sole heir to the crown of Castile. Juan II died in 1479, and Ferdinand succeeded to the throne in January 1479.
In September 1479, Portugal and the Catholic Monarchs of Aragon and Castile resolved major issues between them through the Treaty of Alcáçovas, including the issue of Isabella's rights to the crown of Castile. Through close cooperation, the royal couple were successful in securing political power in the Iberian peninsula. Ferdinand's father had advised the couple that "neither was powerful without the other". Though their marriage united the two kingdoms, leading to the beginnings of modern Spain, they ruled independently and their kingdoms retained part of their own regional laws and governments for the next centuries.