On a winter morning in February 1670, an eighteen-year-old empress lay in the grip of a labour that had come far too early. In her fear she made a bargain with God. If she survived, she would drive the Jews out of Vienna. Her infant son died that very day.

She lived. By the end of that summer her husband had emptied the city’s Jewish quarter, and a church was rising where the synagogue had stood. The woman in that moment of dread had once been a small girl standing at the centre of one of the most famous paintings in the world, Las Meninas. Her name was Margaret Theresa of Spain, and by the age of twenty-one she would be dead.

To understand how she came to make that terrible vow, and why her children kept dying, it is necessary to trace the tangled family tree that shaped her entire life. Her parents were an uncle and a niece. Her father was Philip IV, King of Spain. Her mother, Mariana of Austria, was his own niece, nearly thirty years his junior.

Mariana had not originally been intended for Philip 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 were married in 1649. Margaret Theresa was born on 12 July 1651, in the Royal Alcázar of Madrid.

Over the next ten years her mother gave birth four more times. A sister who lived only a few days. A brother, Felipe Próspero, who died just before his fourth birthday. Another brother, Fernando Tomás, who perished as a small child.

And finally, in 1661, the boy who would become King Charles II, the last Habsburg to rule Spain. Of those five children, only two survived childhood, and Margaret Theresa was one of them. Her own health was fragile too, yet according to her biographer she grew into a lively and intelligent girl. While her brothers died around her, she was immortalised in paint.

In 1656 Diego Velázquez placed her at the centre of Las Meninas, aged five, in a wide silver dress, the royal court arranged around her. Her parents appear only as a dim reflection in a mirror at the back of the room. Three years later Velázquez painted her again, aged eight, in a blue dress. That second portrait did not remain in Spain.

Her father sent it to Vienna as a gift for Leopold. It still hangs there today, in the Kunsthistorisches Museum. Such portraits were how her future husband watched 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, travelled to Madrid to settle the terms, and the betrothal was proclaimed in April 1663. The marriage contract was signed that December. On paper the union was meant to preserve and strengthen the Catholic cause. The contract also provided for a dowry, though that dowry was never paid.

But one clause mattered far more than money. Margaret Theresa’s elder half-sister, Maria Theresa, had been required to renounce her claim to the Spanish throne when she married the King of France. Margaret Theresa was asked to renounce nothing. Her German biographer described this as a sensational political matter, because it meant her right to Spain would travel with her to Vienna.

Her little brother was sickly, and such a claim was immensely valuable. Spain kept her there until he left. Philip IV died in September 1665, and in the following spring, on 25 April 1666, Margaret Theresa was married by proxy in the Alcázar. She was fourteen years old.

Three days later she set out for Vienna, travelling by way of Barcelona, Milan and Trento, and finally over the mountains on the Semmering road. In late November, as her procession drew near, Leopold rode out to Schottwien, below the Semmering Pass, to catch a hidden glimpse of his bride before the official reception. The wedding itself took place in Vienna on 5 December 1666. She was fifteen.

Vienna then staged celebrations that lasted, off and on, for nearly two years. Fireworks displayed their initials, L and M, inside glittering hearts, followed by three hundred rockets forming 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, called The Contest of Air and Water, designed to outshine the festivities of Versailles, with machines that made 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, The Golden Apple, in a new theatre 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 festivities at more than 200,000 thalers. Behind all of it, Margaret Theresa lived in a small Spanish world of her own. Nearly all her secretaries, confessors and physicians were Spanish, and she barely learned German.

Her biographer describes her marriage as an extremely happy one, with the couple sharing a love of music and art. Yet while she was still alive, Leopold kept another option open, a Habsburg archduchess from Tyrol named Claudia Felicitas. 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 reaching four months old, of what the records call catarrh and convulsions. Margaret Theresa decided she knew the cause. According to the Jewish Encyclopedia, she came to believe that the toleration of Jews in Vienna was responsible for the death of her firstborn. 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, Maria Antonia, was born, and she survived. Then, on 20 February 1670, came the labour described at the beginning. The child, a boy named Johann Leopold, was premature and died on the day of his birth. During that delivery Margaret Theresa made her vow, and the new German-language biography, the standard reference on her life, records exactly what it was.

If she survived, she would drive the Jews out of the city and out of the country. That summer the Jewish quarter of Vienna was cleared, and by the end of July 1670 the last families had gone. It became known as the Second Viennese Gesera. In August Leopold laid the foundation stone of a new church on the site of the synagogue, with Margaret Theresa beside him.

The church was dedicated to Saint Leopold, and the entire quarter was renamed Leopoldstadt in the emperor’s honour. It still bears that name today, the second district of Vienna. The five-year-old girl in Las Meninas had grown up to stand at the foundation stone of a church built where a synagogue had stood. 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 issued a patent of protection for the Jews, then turned against them himself. The city council requested the expulsion. The Christian citizens wanted it, fearing competition, and so did clergy such as Bishop Leopold Karl von Kollonitsch.

Leopold went ahead despite the intervention of the Pope. Vienna’s city history wiki says the decisive impulse came above all from the clergy and the citizens, but it places one other name beside the emperor’s, that of his wife. In 2020, on the 350th anniversary, Cardinal Christoph Schönborn also named her among the Catholics whose hostility had led to the expulsion of the Jews. Vienna paid a price.

Within three years the court treasury office reported that the city’s economy had worsened. And if Margaret Theresa believed her vow would protect her children, it did not. In February 1672 she gave birth to a daughter, Maria Anna, who lived for two weeks. Her cause of death is listed as unknown.

That same year Margaret Theresa brought the cult of a Spanish saint, Peter of Alcántara, to Vienna and founded a chapel for him. Its altarpiece shows Margaret Theresa with Leopold and their daughter, the only one of her children who remained alive. 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 taken place.

The figure they used is called the coefficient of inbreeding, 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 Theresa’s mother had a coefficient of 0. 155, the highest of any Spanish royal wife. 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 marriages of uncles and nieces. Leopold had the highest figure among Habsburg emperors, 0. 1568.

Their daughter, the only infant who survived, had a coefficient of 0. 3053, higher than the child of a brother and sister. Margaret Theresa’s own figure was not given in either study. Their earlier study had examined Spanish royal families between 1527 and 1661.

Of 34 children, 10 died before their first birthday, and half were dead before the age of 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, not conclusive, and say these deaths could also have had environmental causes.

Meanwhile Margaret Theresa 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 goitre. Then she developed bronchitis, and according to her German biographer she choked to death.

She died on 12 March 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 regarded her death in her twenty-first year as a stroke of luck for the Austrian line.

Leopold was grief-stricken. He mourned her as “the only Margaret,” Margareta unica, the irreplaceable Margaret. Four months after her death he announced his next wife, and on 15 October 1673 in Graz he married the archduchess from Tyrol. The daughter Margaret Theresa 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 Theresa’s brother died without children. The Spanish Habsburg line came to an end, and wars broke out across Europe over its inheritance.

Margaret Theresa lies in the Imperial Crypt in Vienna, in a pewter coffin resting on statues of crowned double eagles and decorated with skulls. An inscription in Latin is on 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. What generations of intermarriage had done to a Spanish princess and her children was now visible for all to see. Yet the Habsburgs continued the practice, and more than a century later it placed on a throne a man whose story is even harder to believe.