In the Oscar-winning period piece The Favourite, two clever, ambitious ladies-in-waiting in early 18th-century England compete for the favor—and romantic affections—of a mercurial and unstable Queen Anne.

The outlandish, profanity-laden and darkly comic film doesn’t stick to the historical record, however—especially when it comes to the fashion, language or dance styles of the period. But the rivalry in Queen Anne’s court between Sarah Churchill, the formidable Duchess of Marlborough, and her upstart cousin, Abigail Masham, was very real—and very ruthless.

Their power struggle also had important political implications, as England during Anne’s reign was deeply divided between two new political parties, the Whigs and the Tories.

Anne was born in 1665, to James, then Duke of York, and his first wife, Anne Hyde. Though her father converted to Catholicism, Anne and her elder sister, Mary, were raised in the Anglican faith, thanks to the influence of their powerful uncle, King Charles II.

Princess Anne was just eight years old when she first met her future aide, Sarah Jennings, who at 13 had just begun serving as a lady-in-waiting to Anne’s stepmother, then the Duchess of York. The beautiful, intelligent and witty Sarah later married an older army officer, John Churchill, whose impressive military record helped make up for his modest background.

When Anne herself married the handsome Prince George of Denmark in 1683, she made Sarah her second lady of the bedchamber, the first of a number of titles she would bestow upon her close friend. At some point, the two of them made up nicknames for each other that put them on equal social footing (at least privately): Anne was Mrs. Morley, while Sarah was Mrs. Freeman.

When Sarah was away from the palace, she and Anne exchanged a flood of letters. “Tis impossible for you ever to believe how much I love you except you saw my heart,” the princess wrote in one letter, as quoted in Anne Somerset’s biography of Anne. “If I writ whole volumes I could never express how well I love you,” read another. Since Sarah convinced Anne to burn her replies, we’ll never know if she responded in kind, or if (as she herself later claimed) Anne’s adoration was one-sided.

Anti-Catholicism ran rampant in England at the time, and in 1688 the Glorious Revolution pushed Anne’s father, King James II, off the throne in favor of William of Orange, Anne’s cousin and the husband of her sister, Mary. With William and Mary on the throne as co-regents, Anne was next in line to the throne, but was having a great deal of trouble producing an heir. Chronically ill throughout her life, Anne suffered from an array of ailments including severe myopia, gout and possibly lupus.

Tragically, though Anne would become pregnant 17 times during her lifetime, most of her pregnancies ended in miscarriages or stillbirths. Only one of her children, a son, would survive infancy, and he died in 1700. Two years later, upon William’s death, Anne was crowned queen of England, Ireland and Scotland.

As Duchess of Marlborough, Sarah took on several key positions in Queen Anne’s court, including mistress of the robes, keeper of the privy purse and groom of the stole. With the War of the Spanish Succession heating up, John Churchill (now Duke of Marlborough) was named captain-general of the Queen’s army, leading British forces to a series of victories, most notably the Battle of Blenheim in 1704.

Meanwhile, at home, Whigs and Tories clashed bitterly over control of Anne’s government. The Whigs, who had played a key role in the Glorious Revolution, favored a constitutional monarchy, where Parliament had more power than the monarch. The Tories sought to uphold royal power, as well as the traditional dominance of the Anglican Church.

Though Anne’s sympathies lay mostly with the Tories, Sarah was a strong Whig. Under her influence, Anne initially let herself be guided by Marlborough and Lord Treasurer Sidney Godolphin, whose job was basically the equivalent of the British prime minister’s today. As moderate Tories, they sided with the Whigs over the direction of the war in Europe, and put pressure on the Queen to include more Whigs in her government.

Even as Sarah and Anne began to grow apart over their differing political views, a rival for the Queen’s affection entered the scene. In 1704, Sarah had installed her cousin, Abigail Hill, as a woman of the bedchamber in Anne’s court. Abigail’s father had gone broke through speculation, then died, and Sarah sought to help her cousin out. While performing tasks like handing the Queen her clothing and pouring water for her to wash her hands and feet, Abigail offered Anne attention and care without judgment or argument—unlike her domineering cousin.

In 1707, in a sign of their growing closeness, Anne attended Abigail’s secret wedding to Samuel Masham, one of Prince George’s servants. Sarah was furious when she found out about the wedding, as well as the fact that Anne had given Abigail a dowry from the privy purse. By mid-1707, both Sarah and her husband suspected that Abigail was using her influence over the Queen to further the agenda of her powerful Tory cousin, Robert Harley. Though Harley was forced to resign from Anne’s cabinet in early 1708, due to his differences with Marlborough, he continued to advise the Queen through Abigail, whom everyone now knew had replaced Sarah as Anne’s favorite at court.

Losing her influence over the queen, Sarah took increasingly desperate measures, including spreading rumors that Anne and Abigail’s relationship was sexual. In mid-1708, she helped to circulate a ballad about Abigail, written by a fellow Whig, with such lyrics as: “Her secretary she was not / Because she could not write / But had the conduct and the care / Of some dark deeds at night.”

Though in The Favourite, Sarah and Abigail both have closeted sexual relationships with Queen Anne, Somerset and other historians argue that this was unlikely. Anne was devoted to Prince George and was regularly pregnant throughout their marriage. Even at the time when she was growing close with Abigail, Anne shared a bed with and cared for her ailing husband, who died in 1708. She was also known to be quite prudish, due to her strong Christian faith, and her worsening health over the course of her reign can hardly be imagined to have increased her sexual appetite.

As for the undeniably romantic letters Anne wrote to Sarah, which Sarah threatened to release during their conflict over Abigail, such passionate expressions of feeling between female friends would not have been considered particularly unusual at the time.

Whatever her earlier feelings, Anne unceremoniously dumped Sarah in 1711, instructing Marlborough to tell his wife to vacate her rooms in St. James’ Palace and return the gold key of the royal bedchamber (the symbol of her former status).

The Queen made Abigail keeper of the privy purse, and gave her husband a peerage, making him Baron Masham of Otes. As for Abigail’s Tory cousin Robert Harley, he became the Earl of Oxford and served as lord treasurer for most of the last years of Anne’s reign. But Abigail would never hold the same sway over the queen as Sarah had, as this time Anne was more cautious about allowing her new favorite to exert too much influence over her affairs.

In 1714, after years of increasingly ill health, Queen Anne died at the age of 49. She would be the last Stuart monarch, as according to the Act of Succession (1701), the throne passed to the House of Hanover. With the ascension of King George I, Abigail and Masham were removed from the palace, and Abigail was even accused of stealing some of the Queen’s jewels. She received help from an unlikely source: Sarah Churchill, who was said to have commented: “I believed Lady Masham never rob’d anybody but me.”

As for Sarah herself, she went on to publish her memoirs, which gave her the upper hand in how history would remember Queen Anne, her reign and especially the power struggle with Abigail Masham. In her long life—she would live to be 84—the Duchess of Marlborough would become the richest woman in England, and one of the richest in the world, with a fortune worth some £4 million, or close to £1 billion today. She also produced one of England’s most celebrated lineages, counting among her descendants not only Winston Churchill but Lady Diana Spencer, better known as Princess Diana.