To understand this piece of history, we must go back to the reign of Henry IV of Castile and his second marriage to Queen Juana of Portugal, as their daughter, known as "Juana la Beltraneja," triggered a civil war between those who supported her as the successor to the throne of Castile and those who favored the king's stepsister, later known as "Isabella the Catholic." Isabella's supporters believed that the supposed daughter of the monarch was not actually his, but rather of a member of his court named Beltrán de la Cueva. As the story goes, we know that Isabella of Castile eventually secretly married Ferdinand of Aragon, uniting a myriad of territories under their rule: Castile, Aragon, Navarre, Granada, the American possessions, etc., leaving them all to their daughter Juana, the heir to the throne, discredited by alleged madness and wed to Philip of Austria. At this point, we can understand how their son, the future Emperor Charles, became the first monarch of the House of Austria.