Charles V, Holy Roman Emperor
Spanish kingdoms

In the Castilian Cortes of Valladolid in 1506 and of Madrid in 1510, Charles was sworn as the Prince of Asturias, heir-apparent to his mother Queen Joanna. On the other hand, in 1502, the Aragonese Corts gathered in Zaragoza and pledged an oath to Joanna as heiress-presumptive, but Alonso de Aragón, Archbishop of Zaragoza, (an illegitimate son of King Ferdinand) expressed firmly that this oath could not establish jurisprudence, that is to say, modify the right of the succession, except by virtue of a formal agreement between the Corts and the King. So, upon the death of King Ferdinand II of Aragon, on 23 January 1516, Joanna inherited the Crown of Aragon, which consisted of Aragon, Majorca, Catalonia, Valencia, Naples, Sicily and Sardinia, while Charles became governor general. Nevertheless, the Flemings wished Charles to assume the royal title, and this was supported by Emperor Maximilian I and Pope Leo X.

Thus, after the commemoration of Ferdinand II's obsequies on 14 March 1516, Charles was proclaimed king of the crowns of Castile and Aragon jointly with his mother. Finally, when the Castilian regent Cardinal Jiménez de Cisneros accepted the fait accompli, he acceded to Charles's desire to be proclaimed king and imposed his instatement throughout the kingdom. Charles arrived in his new kingdoms in autumn of 1517. Jiménez de Cisneros came to meet him but fell ill along the way, not without a suspicion of poison, and he died before reaching the King.

Due to the irregularity of Charles assuming the royal title while his mother, the legitimate queen, was alive, the negotiations with the Castilian Cortes in Valladolid (1518) proved difficult. In the end Charles was accepted under the following conditions: he would learn to speak Castilian; he would not appoint foreigners; he was prohibited from taking precious metals from Castile beyond the Quinto Real; and he would respect the rights of his mother, Queen Joanna. The Cortes paid homage to him in Valladolid in February 1518. After this, Charles departed to the Crown of Aragon.

He managed to overcome the resistance of the Aragonese Cortes and Catalan Corts, and he was recognized as King of Aragon and Count of Barcelona jointly with his mother, while his mother was kept confined and could only rule in name. The Kingdom of Navarre had been invaded by Ferdinand of Aragon jointly with Castile in 1512, but he pledged a formal oath to respect the kingdom. On Charles's accession to the Spanish thrones, the Parliament of Navarre (Cortes) required him to attend the coronation ceremony (to become Charles IV of Navarre). Still, this demand fell on deaf ears, and the parliament kept piling up grievances.

Charles was accepted as sovereign, even though the Spanish felt uneasy with the Imperial style. Spanish kingdoms varied in their traditions. Castile had become an authoritarian, highly centralized kingdom, where the monarch's own will easily overrode legislative and justice institutions. By contrast, in the Crown of Aragon, and especially in the Pyrenean Kingdom of Navarre, law prevailed, and the monarchy was seen as a contract with the people. This became an inconvenience and a matter of dispute for Charles V and later kings since realm-specific traditions limited their absolute power. With Charles, the government became more absolute, even though until his mother died in 1555, Charles did not hold absolute power in the country.

Soon resistance to the Emperor arose because of heavy taxation to support foreign wars in which Castilians had little interest and because Charles tended to select Flemings for high offices in Castile and America, ignoring Castilian candidates. The resistance culminated in the Revolt of the Comuneros, which Charles suppressed. Comuneros released Joanna and wanted to depose Charles and support Joanna to be the sole monarch instead. While Joanna refused to depose her son, her confinement would continue after the revolt to prevent possible events alike. Immediately after crushing the Castilian revolt, Charles was confronted again with the hot issue of Navarre when King Henry II attempted to reconquer the kingdom. Main military operations lasted until 1524, when Hondarribia surrendered to Charles's forces, but frequent cross-border clashes in the western Pyrenees only stopped in 1528 (Treaties of Madrid and Cambrai).

After these events, Navarre remained a matter of domestic and international litigation still for a century (a French dynastic claim to the throne did not end until the July Revolution in 1830). Charles wanted his son and heir Philip II to marry the heiress of Navarre, Jeanne d'Albret. Jeanne was instead forced to marry William, Duke of Julich-Cleves-Berg, but that childless marriage was annulled after four years. She next married Antoine de Bourbon, and both she and their son Henry of Navarre would oppose Philip II in the French Wars of Religion.

After its integration into Charles's empire, Castile guaranteed effective military units and its American possessions provided the bulk of the empire's financial resources. However, the two conflicting strategies of Charles V, enhancing the possessions of his family and protecting Catholicism against Protestant heretics, diverted resources away from building up the Spanish economy. Elite elements in Spain called for more protection for the commercial networks, which were threatened by the Ottoman Empire and Barbary pirates. Charles instead focused on defeating Protestantism in Germany and the Netherlands, which proved to be lost causes. Each hastened the economic decline of the Spanish Empire in the next generation. The enormous budget deficit accumulated during Charles's reign, along with the inflation that affected the kingdom, resulted in declaring bankruptcy during the reign of Philip II.
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