Dynastic Union

The Beginning
In the 15th century, the most important among all of the separate Christian kingdoms that made up the old Hispania were the Kingdom of Castile (occupying northern and central portions of the Iberian Peninsula), the Kingdom of Aragon (occupying northeastern portions of the peninsula), and the Kingdom of Portugal occupying the far western Iberian Peninsula. The rulers of the kingdoms of Castile and Aragon were allied with dynastic families in Portugal, France, and other neighboring kingdoms.

The death of King Henry IV of Castile in 1474 set off a struggle for power called the War of the Castilian Succession (1475–79). Contenders for the throne of Castile were Henry's one-time heir Joanna la Beltraneja, supported by Portugal and France, and Henry's half-sister Queen Isabella I of Castile, supported by the Kingdom of Aragon and by the Castilian nobility.

Isabella retained the throne and ruled jointly with her husband, King Ferdinand II. Isabella and Ferdinand had married in 1469 in Valladolid. Their marriage united both crowns and set the stage for the creation of the Kingdom of Spain, at the dawn of the modern era. That union, however, was a union in title only, as each region retained its own political and judicial structure. Pursuant to an agreement signed by Isabella and Ferdinand on January 15, 1474, Isabella held more authority over the newly unified Spain than her husband, although their rule was shared. Together, Isabella of Castile and Ferdinand of Aragon were known as the "Catholic Monarchs" (Spanish: los Reyes Católicos), a title bestowed on them by Pope Alexander VI.