In the midst of England’s Hundred Years’ War with France, it secured a shockingly long-lasting alliance with Portugal that would begin in the 14th century and last until the Napoleonic Wars of the 19th. At its center was the marriage of Philippa of Lancaster, granddaughter of England’s King Edward III, to Portugal’s King Juan I. Their children would become known as the “Illustrious Generation” and lead the country into one of its most profitable and historically significant eras.

Philippa was born at Leicester Castle on March 31, 1360, the eldest child of her parents, John of Gaunt and Blanche of Lancaster. John was the fourth son of Edward III and his wife, Philippa of Hainaut, for whom Philippa was named, while Blanche was the younger daughter of the enormously wealthy and powerful Henry of Grosmont, Duke of Lancaster. Blanche’s father would die a year after Philippa’s birth, at which point her father was made Duke of Lancaster in a new creation by King Edward.

Though John and Blanche would have seven children in all, only three would survive infancy and Philippa was joined in the nursery by a younger sister, Elizabeth, on February 21, 1364, and a younger brother, Henry of Bolingbroke, on April 3, 1367. Unfortunately Blanche wasn’t long for the world and she died on September 12, 1368, shortly after giving birth to a short-lived daughter, Isabel.

John would remarry on September 21, 1371 to Constance of Castile, a claimant to the Castilian throne who had been boxed out of succeeding the throne from her father, Peter the Cruel, by her illegitimate half-uncle, Henry II. The marriage was political – Constance needed a husband to lead military intervention on her behalf, while John, if he was successful, would become king of Castile. Indeed, by 1372 the two were styled in England as the king and queen of Castile.

The marriage was clearly only political for a second reason, which was that John had begun what would become a decades-long affair with a widow by the name of Katherine Swynford. Their eldest son, John Beaufort, was born the year after his eldest daughter by Constance, who was intriguingly named Katherine.

Philippa would have known Katherine well, possibly better than her stepmother since Katherine was given the position of governess to his older children, allowing her to oversee their education while she raised her illegitimate children, all given the surname of “Beaufort.” It’s unclear what Philippa made of all of this, but having known Katherine since her childhood, all evidence points to the two women having a pleasant relationship.

And certainly she had enough time to do so, for Philippa wasn’t married until age 27, remarkably late for the 14th century. She married King Juan on February 14, 1387 and departed for Portugal, however once there she quickly learned that her new husband had had a mistress of his own for the last decade, as well as two illegitimate children. While Philippa welcomed Juan’s children at court, she quickly saw to it that the mistress, Inês Peres Esteves, was sent away to a convent, perhaps having no wish to play out the role of Constance of Castile in her own life.

Her marriage helped to secure the Anglo-Portuguese alliance, ratified with the Treaty of Windsor in 1386. At its start, Portugal’s connection with John of Gaunt was considered circumspect with suspicions that he might try to seize the country given his efforts in nearby Castile – indeed, the Treaty of Windsor was in response to Castile’s recent alliance with France. Instead, the peace would be a success and the remnants of this agreement would carry as far as World War I, helping to dictate Portugal’s neutrality in the European crisis.

In the same year of Philippa’s marriage, John would be forced to enter Portugal after a failed campaign in Leon.It was then that he finally came to terms with Henry II of Castile, agreeing to marry Katherine, his daughter by Constance, with Henry’s eldest son and heir. The marriage treaty was ratified on July 8, 1388 and ended Spain’s portion in the Hundred Years’ War. In August 1388 Katherine of Lancaster confirmed her willingness to the match and on September 19th she and the future Henry III of Castile were married. Ironically, Katherine would become the great-grandmother of Katherine of Aragon, the first of Henry VIII’s wives.

For her own part, Philippa was an excellent Medieval queen. Though she claimed it was outside of her purview to interfere in politics, it’s clear that she kept a close watch on the political situation in England, particularly from 1399 on, when her younger brother deposed their cousin, Richard II, and ascended the throne as Henry IV. She has also been credited with spearheading Portugal’s siege of Ceuta, which, though she didn’t live to see its completion, was a success within a month of her death.

Philippa and Juan would  have nine children of their own, six of whom lived into adulthood, including Prince Edward, who would succeed his father to the throne, and Prince Henry, who became known as “Henry the Navigator” for his contributions to the Age of Exploration in the 15th century. Her only daughter, Isabella, would go on to marry Philip, Duke of Burgundy as his third wife in 1430 and play a pivotal role in the final stages of the Hundred Years’ War and England’s Wars of the Roses.

Philippa fell ill with the plague in the summer of 1415 and traveled from Lisbon to Sacavém, where she died surrounded by her husband and children. Juan would go on to live until 1433, never remarrying, while her son would become known as Edward the Philosopher, renowned for his learning, and admitted to England’s Order of the Garter. Philippa is buried at the Batalha Monastery in Leiria.