/u/spyhops

To understand why an infamously xenophobic Crown of Castile ended up supporting a Bourbon Duc d'Anjou to inherit the crowns of Spain as Philip V, we have to first understand the reign of Charles II the last Habsburg king of Spain and how the succession crisis came about.

First on the succession crisis, it was already known that Charles II was "incapable" and thus not likely to produce an heir. This brought about negotiations between claimants and their supporters that disrespected Charles II and his wish that his inheritance be kept intact.

In that period, Spain underwent a succession of crises in governance, not only due to the decline of the final years of Philip IV, but also due to the incapacity of Charles II. As Philip IV neared death, his illegitimate son Don Juan José maneuvered to place himself at the center of government, summoning the romantic memory of legendary Don Juan of Austria, victor of Lepanto. Philip IV knew better, so he moved to exclude Juan José from government and instead set up a Junta de Gobierno to advise the queen until Charles II was to come of age. This Junta was selected to represent member states so that their interests would be represented.

However, this meant competing interests incapacitated this system, and led to serious decline in royal authority in which Castile's former authority was reduced to central-in-name-only. Throughout all this, Juan José succeeded in building a power base in Aragon, from which he raised an enthusiastic following and entered Madrid in triumph in 1669. This was a remarkable milestone, as for the first time a claimant whose power base was in a lesser province (Aragon) could dominate in the central province (Castile). Either due to his own lack of confidence, or lack of political skill, or fear of a civil war, Juan José agreed to a power-sharing agreement in which he became viceroy in Aragon. A new favorite, Valenzuela, rose in government despite his low birth, fomenting jealousy from Castile's elite ruling class. When he was deposed in 1677, Don Juan José once again marched on Madrid to finally take direct control, only to die in 1679. The queen mother's faction took control and taking into account the wish of the nobles of Castile, another junta was set up that was weak, corrupt, and ineffective.

All that happened during a time of succession crises as Charles II was married off again and again with the hope that he could produce an offspring. The second attempt was in wedding Charles II to Mariana de Neuburg, an Austrian and daughter of the Elector Palatine, in 1690. Clearly, politics was more important than genetic vitality, as this marriage did nothing to help with the latter. Her arrival in Spain brought along a powerful and power-hungry retinue of Austrians, who colluded with some Spanish nobles to take control of government. This led to enmity from the Council of Castile, who demanded the resignation of this Austrian Party in 1694. The resulting power struggle largely wet in favor of the Queen, but the leading figures of the Council of Castile was able to retain their power.

So, there was a lot of enmity between the nobles of Castile and the Austrian Party. In addition, in this period the Austrian Habsburgs started to propose the idea that they could obtain the Spanish domains in Italy. This meant a significant blow to Charles II's dream that his inheritance could be kept intact. The combination of the two meant that both the nobles and Castile, and Charles II, agreed to declare Anjou to be his successor.

And that's how Castile ended up supporting Anjou, who became Philip V.