Fernando and Isabel

The Catholic Monarchs is the joint title used in history for Queen Isabella I of Castile and King Ferdinand II of Aragon. They were both from the House of Trastámara and were second cousins, being both descended from John I of Castile; on marriage they were given a papal dispensation to deal with consanguinity by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was eighteen years old and Ferdinand a year younger. It is generally accepted by most scholars (John Elliott being an English-speaking example) that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella. Some newer historical opinions propose that under their rule, what later became Spain was still a union of two crowns rather than a unitary state, as to a large degree Castile and Aragon remained separate kingdoms, with most of their own separate institutions, for decades to come. The court of Ferdinand and Isabella was constantly on the move, in order to bolster local support for the crown from local feudal lords.

The title of "Catholic King and Queen" was bestowed on Ferdinand and Isabella by Pope Alexander VI in 1494, in recognition of their defence of the Catholic faith within their realms.

"Catholic monarchs" or "kings" can also be used in a generic sense (e.g., "the Pope had authority over Catholic monarchs..."); the particular or generic use can be distinguished from the context.

Isabella was named heir to the throne of Castile by her half brother Henry IV of Castile in the Treaty of the Bulls of Guisando. She became Queen of Castile in 1474. Her niece, Joanna of Castile, attempted to gain the throne by bringing in the foreign help of Afonso V of Portugal, leading to the War of Castilian Succession. More recently, some speculate that Joanna was the legitimate successor, though Isabella was able to portray herself as such. Isabella's supporters came out ahead in good part due to Aragon's support through Ferdinand, and she officially won in 1479 via the Treaty of Alcáçovas. Ferdinand became the King of Aragon in 1479. 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 few centuries.

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