Carlos II ruled in what was a very trying time for the Spanish Habsburg monarchy. The rule of his father, Philip IV, had lurched from disaster to disaster during the latter part of his reign. Although Spain was a global power at the start of the seventeenth century, there was an increasing perception within the Spanish elite that Spanish power rested on a highly precarious foundation. The efforts of Philip IV's chief minister, the Count-Duke of Olivares, to right the Spanish ship of state came to naught in the many disasters of 1640. Olivares's main concern was to create a "Union of Arms" which would bind all the important components of the Habsburg dominions in a common purpose.

In this period, Spain itself was not really "Spain" but rather what the eminent historian of Spain J. H. Elliott termed a composite monarchy. The Habsburg family ruled a complex entity of separate kingdoms and principalities like Aragon, Castile, Portugal, or Navarre. In exchange for local privileges, the nobility and elites of these regions delegated to the Habsburgs a degree of executive control. This tension between the Habsburg center and its various dominions had been a consistent problem for the Habsburgs with periodic revolts in defense of local privileges being quite common. Olivares's Union of Arms exacerbated these tensions, culminating in two major revolts in Portugal and Catalonia in the 1640s. Both revolts were long and protracted with the Portuguese revolt being successful. These revolts came on top of Spanish military reverses in the Low Countries and Italy and a resurgent France under the new Bourbon dynasty. Moreover, the Olivares regime had proven woefully unable to manage the Spanish economy, leading to massive inflation.

Within this context of persistent defeats and setbacks, the febrile and inbred Carlos II in 1665 posed something of a dubious benefit to Habsburg power. The infante had difficulty speaking at an early age and his deformed jaw was an impediment to his speaking. The result of his lack of education and infirmities was that he often repeated his counselors' advice and seldom deviated from them. This tractability counter-intuitively helped stabilize Spain. After the disastrous results of Olivares's efforts at reform, few of Spain's elites had much stomach for any wide-reaching program of economic and political initiatives aimed at centralizing Habsburg power. A more energetic monarch might have been tempted to try and enact needed reforms after the disastrous 1640s, but the lesson of Olivares was that such reforms often ran counter to the entrenched interests of the Spanish grandees. After reaching his majority, Carlos II's infirmity led to him often delegating authority to royal favorites. Within the complex politics of the court, various power centers arose around different figures. Carlos II's mother, Mariana of Austria, and her favorite Fernando de Valenzuela, were leaders of an early faction, but they increasingly locked horns with a faction led by Philip IV's bastard son, John of Austria.

This factionalism based upon royal favorites had been part and parcel of Spanish royal politics since the accession of Carlos I/V in the sixteenth century, but the Carlos II's infirmity led to a formalization of the process of delegating authority to a valido (favorite). Valenzuela was arguably Spain's first true Prime Minister as he possessed formal legal authority to attend conferences of the various executive components of the Spanish government. this was something quite different that the rather nebulous positions of Validos like Olivares or the Duke of Lerma (Philip III's Valido) whose exact function and place in the machinery of government was unclear. Older historiography of Spain tended to emphasize that the Spanish factionalism reached its apogee under Carlos II and its patronage system further sapped Spanish power, but recent work by Christopher Storrs has contended that despite the viciousness of domestic infighting, these factions often tried to rule responsibly when they had achieved power. These factions often tried to manage Spain's decline and preserve whatever power it had left abroad and within Spain itself. For his part, Carlos II was deeply affected by his father's will, which charged him to preserve Habsburg power. To a certain degree, Carlos II was successful in this regard. Habsburg authority remained intact in Catalonia, Aragon, Castile, and in its overseas dominions. This was a far cry from the 1640s when it seemed like the entirety of the Spanish Empire was coming apart at the seams. However, Carlos II's various governments were never able to eliminate the power of local grandees, which imposed a limit upon the ability of the crown to raise resources and mobilize Spanish power. So while Spain's geostrategic power did not significantly diminish under Carlos II, it could not expand either. This cemented a perception within Europe that Spain was a declining power and the Carlos II's infirmity did little to change this opinion.

As for Carlos II's inbreeding, this extreme case of a near circular family tree was the byproduct of dynastic accident and geostrategic timing. The Habsburgs had inherited Spain by chance with the timely death of Ferdinand and Isabella's only surviving son Prince John in 1497 and eldest daughter Isabella in 1498. The royal line then fell to Juana I, who had been married to the Habsburgs, who were the emperors of the Holy Roman Empire and rulers over the Low Countries. Juana's son, Carlos I (also known as Charles V), was both King of Spain and Emperor of the HRE, but this global empire was too much for a central figure to administer, and Carlos I divided his patrimony with his son Philip II ruling over Spain and the Low Countries while his brother would head the HRE. This created a split between the Habsburg line with the Austrian and Spanish branches of the family heading distinct political entities. Marriage between these two branches was a means to ensure a renewal of common purpose and shared geostrategic interests. The Spanish Habsburgs alternated between matches with France and Austria. The advent of Protestantism further complicated matters as not only did it eliminate an alternative great power, England, as a marriage match, but the needs to preserve Catholicism in Central Europe pushed for more matches between the two branches of the family. While many European monarchs were aware of this interbreeding, the Habsburg jawline entered into public knowledge, the deleterious effects of this interbreeding were not immediately apparent until Carlos II. Philip IV had married Elisabeth of France, but he intended to marry his son Balthasar Charles to his niece Mariana of Austria. But the untimely death of both his son in 1646 and French wife in 1644 meant that Philip needed an heir and war with France prevented him from another French match. So Philip IV married his own niece, Mariana, and the result of this union was the ill-begotten Carlos II in 1661 (for an interesting paper on Habsburg genetics- see here ). Carlos II's various infirmities were apparent at an early age, but the fact that he was the only surviving legitimate male heir to Philip IV meant that the court elite had little choice but to accept him as ruler. In this case, the need for stability and certainty trumped the desire for an energetic ruler. The hope was that Carlos II could produce an heir with first the French Marie Louise of Orléans and later Palatinate princess Maria Anna of Neuburg, but it is likely that Carlos II was sterile because of his inbreeding.

The popular attention today devoted to exploring the grotesqueness of Carlos II's inbreeding often misses the forest for the trees in what how inbreeding brought about the end of the dynasty. The Habsburg system of governance was able to mitigate much of the problems Carlos II's mental problems created. The Spanish court etiquette and protocol had become so staid since Philip II that a mentally-deficient monarch could be worked around. The bigger problem for the Habsburg monarchy that inbreeding created was sterility. While the Spanish system could handle an inbred monarch, a monarch without a clear heir was something different all together.

Sources

Elliott, J. H. Imperial Spain, 1469-1716. New York: St. Martin's Press, 1964.

_. Spain, Europe & the Wider World, 1500-1800. New Haven: Yale University Press, 2009.

Lynch, John. The Hispanic World in Crisis and Change, 1598-1700. Oxford, UK: Blackwell, 1992.

Storrs, Christopher. The Resilience of the Spanish Monarchy, 1665-1700. Oxford: Oxford University Press, 2006.

You may be interested in this previous question, which provides sources on Charles II's modern diagnosis of, among other things, Combined Pituitary Hormone Disorder, which can impact cognitive functioning. A brief review of the literature on cognitive impairment in Combined Pituitary Hormone Deficiency turned up a 2016 study which found cognitive abilities in affected adults to be in the range of 46-116, suggesting that for at least some severely affected adults, cognitive functioning could be in the Moderate Mental Retardation range. The mean total IQ score was 81, which is in the Low Average range. Of course, these are modern categorizations and it is impossible to conclusively forensically evaluate Charles II.

Sources: Warmuz-Stangierska, I. et. al (2016). Cognitive function in patients with childhood-onset combined pituitary hormone deficiency not treated with growth hormones. Journal of Medical Science, 85(3). https://doi.org/10.20883/jms.2016.116

American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.). Washington, DC: Author.

To tag onto this question. A lot of what I've read about Charles I/V says that the people caring for him did not make him attend any type of schooling for fear it would 'over tax' him.

I feel like this would have effected a lot of how mentally deficient he was viewed, because it seems like he was a mix of someone who was not completely in control of his mental faculties, but also extremely uneducated and indulged as a child.

Am I on the right track with this kind of thinking? As a parent, this makes me think that he would have been pretty much a spoiled brat king and was hindered immensely by the decision to not have him attend school or learn self care items.