This guy. One of the most perplexing Habsburgs (or indeed, European dynasts from any clan), Karl was the closest thing continental Europe had to a “universal” monarch between the heyday of Rome and the exploits of Napoleon almost two millennia later.

He wasn’t the worst of the Habsburgs (an honor shared between his son, Philip II, and his grandson, Ferdinand II), and he’s hard to thoroughly dislike (he often displayed a disarming sense of humor about his august self)—but he’s equally challenging to admire. Contemporaries loved him and hated him to about equal degrees. Today, his reign comes across as a tremendous missed opportunity: so much potential and so little to show for it in the end.

He was perhaps the greatest beneficiary of the Habsburg’s marriage program. In true Habsburg style, he inherited a smorgasbord of territories: the Netherlands from his father, one half of Spain (Castile) from his mother, the other half of Spain (Aragon) from one grandfather, and another grandfather left him all of the Habsburgs’ lands in Central Europe. But what would be the fun without still more exciting prizes to take home? Each of the Spanish legacies came with a bonus: two-thirds of Italy (courtesy of Aragon) and the ravaged lands of the Aztec and Incan Empires (courtesy of Castile’s genocidal search for gold in the Americas).

Most folks would celebrate this sort of luck or at least indulge in an occasional Mr. Burns-style chortle. Karl just looked permanently constipated by the task of keeping rivals, foreign and domestic, in check, managing the jealousies and conflicting expectations of his disparate realms, and passive-aggressively entreating his family to shoulder much of the burden. 

The self-consciously devout emperor, who saw himself as a divine instrument, also faced a challenge on the religious front. Whether you called it a call for ecclesiastic reform or an outbreak of heresy, many of his German subjects challenged the Christian status quo. A neurotic Thuringian monk decided to work out his issues by nailing a manifesto on a church door (the late-medieval analog to Reddit) and kicking off the Protestant Reformation. This troublemaker was Martin Luther, who in earlier times would have been quickly burned alive (like all previous dissenters against Christian orthodoxy). Instead, this firebrand enjoyed the protection of powerful German princes, so his repudiation of papal authority and conventional theology thrived.

Karl would have doubtless preferred the option of public immolation to deal with this irritating malcontent. But international developments made a righteously vigorous crackdown inadvisable.

For starters, the scale and strategic position of Karl’s vast holding made his neighbors—the French specifically—very nervous. France had good reason to fret: it was now encircled by lands owned or controlled by the Habsburgs. A generation earlier, there had been five major European powers: France, England, Burgundy, Spain, and the Empire. The last three were now under Habsburg suzerainty, placing the French kingdom in peril. For the next two-and-a-half centuries, French diplomacy and war-making would be dominated by Paris’s mission to drive back (or at least contain) the sprawling Vienna-Madrid axis.

If Christian threats alone weren’t enough to put Karl off his feed, he also faced the burgeoning Muslim power of the Ottoman Empire in the east. The Ottomans fielded the most advanced army of the age, combining innovations in gunpowder weapons and heavy artillery to outmatch their Christian foes. They’d already swallowed the Balkan peninsula; now, they set their sights on Hungary, Croatia, and Vienna itself. In desperate need of troops to repel French and Turkish invades, Karl traded limited toleration of Protestantism for feet on the ground, allowing the Lutheran “heresy” to spread without hindrance.

All of this took its toll on Karl’s peace of mind. Initially, he’d trusted God to intervene to defeat all his enemies. The pure luck of the draw meant that it happened on some occasions yet not on others. Although he never relinquished the belief that he was an instrument of the (respectably orthodox) Christian divine, even this narcissistic control freak recognized he’d come to the end of his run.

Beset by scheming Valois kings, cranky Lutherans, even crankier Calvinists, and endlessly squabbling local elites from Bruge to Barcelona, Karl threw in the towel in 1556. I believe he was the only emperor other than Diocletian in ancient Rome to willingly abdicate his crown, peacefully transfer his powers to his heirs, and then retire to semi-obscurity. It’s worth noting that both men died in their beds.

Before taking his leave, Karl split his realm in two. His brother, Ferdinand, got the old Habsburg heartland and the HRE imperial title; his son, Phillip, got Spain, the Netherlands, and southern Italy. Two Habsburg branches emerged: the Spanish and the Austrian. They didn’t always act in concert, nor were their interests necessarily aligned. But together, they continued to menace the rest of Europe—and offered exciting new opportunities for the Habsburgs to interbreed.

Karl retired to custom-built apartments at the Jeronimite monastery of Yuste, deep in Spain’s Gredos mountains. The repurposed dormitory included a bedroom that “from the bed one would see the principal altar” in the church—a final example of Karl’s penchant for mingling religious devotion with stupendous narcissism.

The prepared quarters proved inadequate (and uncomfortably chilly), so after the ex-emperor’s arrival, his quarters underwent significant renovation and expansion. Karl did not derive much benefit from these, dying only a year and a half after he made Yuste his home. The cause was what contemporaries called “quatrain fever”—we know it today as the most severe strain of malaria, Plasmodium falciparum, transmitted by the Anopheles mosquito. The insect that delivered the fatal bite most likely spawned in the fish ponds Karl had requested be built near his rooms.