In February 1670, an eighteen-year-old empress went into labor far too early and offered God a bargain. If she survived, she would expel the Jews from Vienna. Her infant son died that same day. She lived, and before that summer was over, her husband had cleared the city’s Jewish quarter, and a church was built where the synagogue had stood.

I have seen this woman before. She is the little girl standing in the center of Las Meninas. Her parents were an uncle and a niece. Her husband was her uncle, and by the time she was twenty-one, she was dead.

And calling him her uncle is the polite way of putting it. Her husband, Emperor Leopold I, was the brother of her mother. But Leopold’s mother was the sister of her father, which made him her cousin as well. If you are trying to work out whether that makes her her own cousin, the answer is no.

It only means the man she married was two of her relatives at once. Her parents’ relationship was just as tangled. Her father was King Philip IV of Spain. Her mother, Mariana of Austria, was his niece, nearly thirty years his junior.

Mariana had not been meant for him at all. She had been betrothed to his son, Prince Balthasar Charles, and when the boy died in 1646, leaving Spain without a male heir, the king asked for her hand himself. They married in 1649. Madrid, July 1651.

Margaret Theresa was born in the Royal Alcázar on the twelfth. Over the next ten years, her mother gave birth four more times. A sister who lived only a few days. A brother, Philip Prospero, who died just before his fourth birthday.

Another brother, Ferdinand Thomas, who died as a small child. And finally, in 1661, the boy who would become King Charles II, the last Habsburg ruler of Spain. Of those five children, only two survived childhood, and Margaret Theresa was one of them. Her own health was fragile too, but according to her biographer, she grew into a lively and intelligent girl.

While her brothers were dying, she was being immortalized in paint. In 1656, Diego Velázquez placed her at the center of Las Meninas, age five, in a wide silver dress, the royal household arranged around her. Her parents appear only as a small reflection in a mirror at the back of the room. Three years later, he painted her again at age eight, in a blue dress, and that painting did not stay in Spain.

Her father sent it to Vienna as a gift for Leopold. You can still see it there today in the Kunsthistorisches Museum. Paintings like these were how her future husband was able to watch her grow up. And he had every reason to watch closely.

In April 1660, Spain promised her to him in marriage. Leopold’s ambassador, Count Franz Eusebius Pötting, went to Madrid to settle the terms, and the engagement was announced in April 1663. The marriage contract was signed that December. On paper, the marriage was meant to preserve and strengthen the Catholic cause.

The contract also stipulated a dowry, but that dowry was never paid. There was, however, one clause that mattered far more than money. Her older half-sister, Maria Theresa, had been required to renounce her claim to the Spanish throne when she married the King of France. Margaret was not asked to do so.

Her German biographer described this as a political sensation, because it meant her claim to Spain would travel with her to Vienna. Her little brother was sickly, so such a claim was worth a great deal. Spain kept her until he left. Philip died in September 1665, and the following spring, on April 25, 1666, she was married by proxy in the Alcázar.

She was fourteen years old. Three days later, she set out for Vienna, traveling through Barcelona, Milan, and Trent, and finally over the mountains on the Semmering Pass. In late November, as her procession drew near, Leopold rode out to Schottwien, below the Semmering, to catch a glimpse of his bride incognito, before the official reception. The wedding itself took place in Vienna on December 5, 1666.

She was fifteen. Then Vienna held a celebration that lasted, intermittently, for nearly two years. There were fireworks bearing their initials, L and M, inside glittering hearts, followed by three hundred rockets that formed the old Habsburg motto, A E I O U. In January 1667, Leopold rode his horse Speranza in an equestrian ballet in the palace courtyard.

It was called The Struggle of Air and Water, designed to outdo the festivities at Versailles, and it used machinery to make horses and chariots appear to float in the air. For her seventeenth birthday in July 1668, he staged an opera called Il Pomo d’Oro, or The Golden Apple, in a new theater seating five thousand. Leopold wrote some of the music himself, and the production featured elephants on stage. One of his biographers estimated the cost of the wedding celebrations at over 200,000 thalers.

Behind it all, Margaret Theresa lived in her own little Spain inside Vienna. Nearly all her secretaries, confessors, and doctors were Spanish, and she barely learned German. Her biographer describes the marriage as a very happy one, the couple sharing a love of music and art. Even so, while she was still alive, Leopold kept another option open: a Habsburg archduchess from Tyrol named Claudia Felicitas.

But what Vienna wanted from her was a son, and that was going badly. Her first child, Ferdinand Wenzel, was born in September 1667. He died the following January, before he was four months old, of what the records call catarrh and convulsions. And Margaret decided she knew why.

According to the Jewish Encyclopedia, she came to believe that tolerating the Jews of Vienna was the cause of her first child’s death. About six weeks later, a fire broke out in the Hofburg, the imperial palace, and the Jews were blamed for that as well. Falsely. In January 1669, a daughter was born, Maria Antonia, who survived.

Then, on February 20, 1670, came the labor you heard about at the beginning. The child, a boy named Johann Leopold, was premature, and he died on the day he was born. During that birth, Margaret made her vow, and the Neue Deutsche Biographie, the standard German reference on her life, records exactly what that vow was. If she survived, she would drive the Jews out of the city and out of the country.

That summer, the Jewish quarter in Vienna was cleared, and by the end of July 1670, the last families had gone. It is known as the Second Vienna Geserah. In August, Leopold laid the foundation stone for a new church on the site of the synagogue, and Margaret stood beside him. The church was dedicated to Saint Leopold, and the entire quarter was renamed Leopoldstadt, after the emperor.

It still bears that name today. It is the second district of Vienna. Think about that for a moment. The five-year-old in Las Meninas grew up to stand at the foundation stone of a church built where a synagogue had stood.

Now, it would be easy to blame her for all of it, but historians do not. The first expulsion order had already been issued in July 1669, months before her vow. In 1665, after a murder case in the Jewish quarter, Leopold had actually issued a patent protecting the Jews, and then he turned against them himself. The city council had asked for the expulsion.

So had the Christian citizens, who feared competition, and so had clergymen such as Bishop Leopold Karl von Kollonitsch. Leopold went ahead with his decision despite the intervention of the Pope. The city’s history wiki says the decisive impetus came above all from the clergy and the citizens. But it places one other name beside that of the emperor: his wife’s.

In 2020, on the 350th anniversary, Cardinal Christoph Schönborn also named her among the Catholics whose hostility led to the expulsion. Vienna paid a price. Within three years, the court chamber’s financial office reported that the city’s economy had worsened. And if Margaret thought this would protect her children, it did not.

In February 1672, she gave birth to a daughter, Maria Anna, who lived two weeks. Her cause of death is listed as unknown. That same year, Margaret brought the cult of a Spanish saint, Peter of Alcántara, to Vienna and established a chapel for him. Its altarpiece depicts Margaret with Leopold and their daughter, the only one of her children who remained alive.

So was God punishing this family, or was something else killing these children? Two Spanish geneticists, Francisco Ceballos and Gonzalo Álvarez, reconstructed the Habsburg family tree in studies published in 2009 and 2013 to measure how much inbreeding had occurred among them. The figure they used is called the inbreeding coefficient: the probability that a person inherits two identical copies of a gene from the same ancestor, one from each parent. For the child of first cousins, the coefficient is 0.

0625. For the child of a brother and sister, it is 0. 25. Margaret’s mother had a coefficient of 0.

155, the highest of any Spanish queen consort. Her brother’s coefficient was 0. 254. This had accumulated over generations because the same distant ancestors kept appearing on both sides of the tree, along with the uncle-niece marriages.

Leopold had the highest figure among Habsburg emperors, at 0. 1568. Their daughter, the only surviving infant, had a coefficient of 0. 3053.

That is higher than the child of a brother and sister. Margaret’s own figure is not given in either study, so we will not try to guess it. Their earlier study covered Spanish royal families between 1527 and 1661. Of 34 children, 10 died before their first birthday, and half were dead before age ten.

In ordinary Spanish villages of the time, infant mortality ran to about one in five. The researchers are cautious about what this proves. They call the evidence indirect and not conclusive, and they say these deaths could also have had environmental causes. Meanwhile, Margaret was pregnant again.

That winter, she was twenty-one years old and about four months along. Since childhood, she had suffered from a swelling in her throat, a goiter. Then she contracted bronchitis, and according to her German biographer, she choked to death. She died on March 12, 1673, holding the emperor’s hand.

The records of the Imperial Crypt say she kept talking until the last quarter of an hour. Not everyone in Vienna mourned. The Swedish envoy, Esaias Pufendorf, wrote that people considered her death in her twenty-first year a stroke of luck for the Austrian line. Leopold was grief-stricken.

He mourned her as the “only Margaret,” the irreplaceable one. Four months after her death, he announced his next wife. On October 15, 1673, in Graz, he married the archduchess from Tyrol. The daughter Margaret left behind did not live long either.

She married the Elector of Bavaria and died in 1692 at the age of twenty-three. Her son, Joseph Ferdinand, was named heir to the Spanish crown, but he died in 1699 at the age of six. The following year, Margaret’s brother died without children. The Spanish Habsburg line came to an end, and wars broke out across Europe over the inheritance.

Margaret lies in the Imperial Crypt in Vienna, in a pewter coffin resting on figures of crowned double eagles adorned with skulls. An inscription in Latin runs across it. It says she lived twenty-one years and eight months. It says she preceded three of her children to heaven and left one behind, the little girl, because she resembled her mother.

She did resemble her. More than anyone at her funeral could have imagined. You have now seen what generations of intermarriage did to a Spanish princess and her children.

But the Habsburgs kept at it, and more than a century later, the practice placed on the throne a man whose story is even harder to believe.