When Spain’s Joanna I of Castile was betrothed to Philip of Burgundy in 1496, no one could have predicted that the marriage would eventually pull Spain into the vast Habsburg Empire. At the time, Joanna was not expected to inherit the throne. She was the third surviving child of the Catholic Monarchs, Isabella and Ferdinand, and both of her older siblings stood ahead of her in the line of succession. Her eldest brother, John, had married Philip’s only sister, Margaret, in 1497.
But John died suddenly at just 19 years old. Joanna’s other older sibling, Isabella, and Isabella’s infant son also died shortly afterward, suddenly making Joanna the heir to Spain’s kingdoms. That dramatic turn of events placed Joanna directly in line for the crown, but it also opened the door for those around her to seize power in her name. Philip, ambitious and eager to extend Habsburg influence, took advantage of the situation.
Sources from the period state that Joanna had been declared mentally unfit to rule independently. That declaration gave Philip—and later her father, Ferdinand—the legal grounds to govern in her place. For nearly 50 years, Joanna remained confined in a palace, unable to exercise real authority over her own kingdoms. She died there in 1555.
By that point, her elder son, Charles, had already been elected Holy Roman Emperor, taking the title Charles I in 1519. When Joanna died, Charles briefly became the sole ruler of all of her Spanish territories. The arrangement did not last long. Charles abdicated in 1556 and divided the Habsburg lands between his brother Ferdinand, who took the Austrian territories, and his son, the future Philip II, who became King of Spain.
That division formally cemented Habsburg control over the Spanish kingdoms, a reality that had been set in motion decades earlier by a marriage no one expected to change the course of European history.