On October 7, 1649, in a church in Madrid, a fourteen-year-old girl married her uncle. The bride was Maria Anna of Austria, and the groom was Philip IV, King of Spain, forty-four years old and known at court as the Planet King. The marriage was not an ordinary royal alliance. It was the product of a dynasty’s desperate need to preserve itself, and it would seal the fate of a kingdom and a bloodline.

Maria Anna was born on Christmas Eve, 1634, in Wiener Neustadt, south of Vienna. Her father was Archduke Ferdinand of Austria, who three years after her birth inherited the Holy Roman Empire, and her mother was Maria Anna of Spain, daughter of King Philip III. She was therefore the product of two branches of the House of Habsburg, the Austrian and the Spanish, which had ruled much of Europe since a fateful division of the family inheritance in the early sixteenth century. For generations, the Habsburgs had married within their own family to keep power concentrated, and Maria Anna’s own parents were closely related cousins.

This practice, which historians would later call inbreeding, was meant to preserve the dynasty. In truth, it was quietly undermining it. Maria Anna grew up in Austria during the Thirty Years’ War, a conflict that had ravaged Central Europe since 1618 and consumed her father’s attention. By 1645, as peace negotiations advanced, plans turned to her marriage.

The intended groom was Balthasar Charles, Prince of Asturias, the only son of King Philip IV of Spain. The match was politically ideal, but it also continued the Habsburg tradition of marrying close relatives: Balthasar was Maria Anna’s first cousin, the son of her mother’s brother. An agreement was reached in 1646, and the wedding was set for a few years later. Then fate intervened.

In October 1646, weeks after the betrothal, Balthasar Charles fell ill and died, possibly from smallpox, just before his seventeenth birthday. Spain was left without an heir, and Philip IV, a widower with no surviving male child, faced a succession crisis. His solution was shocking. Since his son could no longer marry Maria Anna, he would marry her himself.

The arrangement was accepted, and after a delay due to her age, the wedding took place when she was fourteen. Maria Anna adapted quickly to life in Madrid. She learned Spanish, navigated the complexities of the court, and became queen consort of an empire that stretched across the Americas, the Philippines, and vast territories beyond. Her marriage to Philip produced several children.

Their first daughter, Margaret Teresa, was sent to Vienna at fourteen and married to her uncle Leopold, Maria Anna’s own brother, a repetition of the family pattern. Two other children died in infancy. A son, Philip Prospero, was born in 1657 but also died. In 1661, Maria Anna gave birth to another son, Charles, who survived and became the last hope for the Spanish line.

Philip IV died in 1665, leaving Spain with a broken empire and a child king. Charles was not yet four years old, and it was immediately clear that he would not be able to rule on his own for many years. Even when he reached maturity, his court would be controlled by powerful nobles. Maria Anna was named regent, and in effect became the de facto ruler of Spain.

But ruling a deeply patriarchal court as a female regent proved difficult. Her authority was constantly challenged by senior nobles who wielded enormous influence. Spain was in severe decline. The gold and silver from the mines of Mexico, Peru, and Bolivia that had fueled its power were running dry.

The country faced economic troubles, domestic unrest, and endless costly wars. One of the longest conflicts was with Portugal, which had rebelled against Spanish rule in 1640 and was backed by France and England. Influenced by her Jesuit adviser, Cardinal Juan Everardo Nithard, Maria Anna decided to cut Spain’s losses. In 1668, she signed treaties ending the war with France and officially recognizing Portugal’s independence.

But Spain’s troubles were not over. Four years later, war broke out again with France, as King Louis XIV sought to seize the Spanish Netherlands, modern-day Belgium and Luxembourg. The struggle for Spain’s survival continued. The crisis at home was even worse.

As Charles II grew, it became clear that he was severely unwell. His body never fully developed. He could barely walk without help, had an extreme Habsburg jaw that made chewing difficult, and his mental faculties were impaired. He aged prematurely, showed dementia-like symptoms in his twenties, and went bald early.

To his contemporaries, he was known as El Hechizado, the Bewitched King, because many believed his afflictions were caused by sorcery. But the likelier cause was the genetic consequence of generations of Habsburg intermarriage. Maria Anna had married her own uncle, and both came from a line that had intermarried for centuries. Some estimates suggest Charles’s inbreeding coefficient was as high as that of a child born to siblings.

Other modern theories point to hormonal deficiencies, skeletal deformities, epilepsy, and possibly an infant infection causing hydrocephalus. Whatever the precise diagnosis, by the 1670s it was undeniable that Charles could never fully rule Spain himself. Charles was declared of age in 1675 at fourteen, ending Maria Anna’s ten-year regency. But factional fighting at court and her son’s obvious incapacity led her to orchestrate a restoration of the regency in 1677.

Her return was short-lived. John Joseph of Austria, the illegitimate son of Philip IV and Charles’s half-brother, seized control. He held power for only two years before his death in 1679, after which Maria Anna reclaimed the regency. This time, she would hold power for the rest of her life.

Her greatest concern was securing an heir. Charles was unfit to rule, but she hoped he might still father a child who could restore the dynasty. In 1679, she arranged his marriage to Marie Louise d’Orléans, niece of Louis XIV. The marriage was miserable and produced no children.

Marie Louise died in 1689, likely from a burst appendix, though rumors of poison circulated. Within months, Maria Anna arranged a second marriage for her son, this time to Maria Anna of Neuburg, chosen because her family was known for fertility. It made no difference. Charles remained without an heir.

These years weighed heavily on Maria Anna. She was anxious about Spain’s decline, her son’s condition, and the future of the Habsburg dynasty. In 1692, her granddaughter Maria Antonia of Austria died at twenty-three, a terrible blow. By the early 1690s, it was painfully clear that Charles would never produce a child.

His health declined rapidly, and many at court expected his death at any moment. Yet he lingered on. Maria Anna would not live to see his final days. In the mid-1690s, she developed what appears to have been breast cancer.

After years of political struggle and failed efforts to secure Spain’s future, she died in Madrid on May 16, 1696, at the age of sixty-one. Her life’s work, preserving the Habsburg dynasty, had failed. Charles II outlived her by only four years. On November 1, 1700, the Bewitched King died at thirty-eight, his two marriages childless.

With his death, the Spanish Habsburg line came to an end. For years, the Spanish court had wrestled with the succession. Maria Anna had favored her nephew, Archduke Charles of Austria, which would have kept the Habsburgs on the Spanish throne. But before his death, Charles II overturned her wishes.

In his final will, he named Philip of Anjou, grandson of Louis XIV of France, as his heir. The choice ignited the War of Spanish Succession in 1701, a conflict that drew in nearly every major European power and lasted thirteen years. It ended with the Treaty of Utrecht in 1713, which confirmed Philip V as King of Spain but stripped Spain of many European territories and permanently shut the Habsburgs out of the Spanish throne.

The consequences of Maria Anna’s marriage and her son’s tragic fate cast a long shadow over Europe for nearly two decades after her death, despite everything she had done to prevent it.