Queen Isabella I of Castile and King Ferdinand II of Aragon were cousins—they were within third degree of cousinage. Under Catholic law, they could not get married. Before their marriage, they had not met, so there was no love between them. Their powerful families decided to get them married because of political reasons—the aim was to unite the dynasties of Castile and Aragon.
At the time of their marriage Isabella was eighteen and Ferdinand was sixteen. The Papal representative in Spain, Antonio Veneris, forged a document which made it possible for Ferdinand to marry his cousin. The document which made the marriage legal was signed in January 1469. Pope Alexander VI granted them the title of “Catholic Monarch.”
With this marriage, Castile and Aragon were united into a formidable kingdom, jointly ruled by Isabella and Ferdinand. During their reign, the Nasrid kingdom of Granada was destroyed, the Reconquista was completed, Spain was aggressively Christianized and united, and the foundation of the Spanish Empire was laid when Columbus was sent on a voyage in which he discovered a new world of fertile land, gold, silver, and slaves.
From the point of view of Western history, the marriage of Isabella and Ferdinand is the most important one. Had they not married, then it is possible there would be no Western power today and history could have taken a different course.