Often times, luck has a big effect in determining what direction history goes. A sudden storm can change the outcome of a battle. A decision by an explorer to take this path through the wilderness instead of that one can change which native tribe the exploration party comes across or if they find a source of fresh water. But the biggest effect luck can have is with dynastic succession. Up until modern times, having a male heir was one of the most important events for a noble family, especially in medieval and renaissance Europe. Even if a son was born, though, there was any number of factors that could lead to the child dying before adulthood and making the line of succession for the family insecure.
This ended up being especially true for the House of Trastámara in Iberia. The dynasty, which had ruled Castile and Aragon for the entirety of the 15th century, finally had the two crowns united by the marriage of Queen Isabella I of Castile and Ferdinand II of Aragon. If they had a son, the two crowns would be fully united under one Trastámara ruler. Due to a very unlikely series of circumstances, the succession would not go as easily as planned.
Isabella and Ferdinand had a total of five children; Isabella, Juan, Juana, Maria, and Catherine. As Juan was their only son, he became the legal heir to both the crown of Castile and the crown of Aragon. In 1496, an alliance with Archduke Maximilian I of Austria was proposed to counter French incursions in Aragonese-owned Naples. As part of the alliance, Juan would marry Maximilian’s daughter Margaret, and Juana would marry his son, Phillip. Juana and Phillip were married on October 2, 1496, and Juan and Margaret married on April 3, 1497. However, the marriage of Juan and Margaret was short as Juan died in October of 1497. Now without a son, Isabella and Ferdinand’s eldest child, Isabella, became the heiress to the two crowns.
Luckily for Queen Isabella and King Ferdinand, Princess Isabella bore a son with her husband King Manuel I of Portugal in 1498. Even though the crowns would not stay in the Trastámara dynasty, they would still be united. And even luckier for King Manuel, his son stood to inherit the crowns of Portugal, Castile, and Aragon and create a united power over the entire Iberian Peninsula and stretching into the Mediterranean and down the African coast. However, Princess Isabella died in childbirth and the son, Miguel da Paz, died two years later in 1500.
Further tragedy befell Ferdinand and Isabella in 1504 when Queen Isabella died, leaving the crown of Castile to Juana. Ferdinand was unwilling to accept the loss of the crown of Castile. He continued minting coins labeled “Ferdinand and Juana, king and queen of Castile” and arranged for the cortes of Castile to declare Juana of mentally ill and appoint him as regent. While it is now accepted that Queen Juana likely had some mental illness after the death of her husband Phillip of Habsburg in 1506, the decision of the cortes and sending Juana to a convent was no doubt a political move to keep Ferdinand in control of Castile.
Meanwhile, King Ferdinand was still aiming to control and hopefully inherit the crown of Castile, or at least keep the crown of Aragon in the Trastámara dynasty. In 1505 Ferdinand embarked on a foreign policy to ally with France and signed the Treaty of Blois. The treaty entailed that Ferdinand would marry Germaine de Foix. If she and Ferdinand can have a son before Ferdinand dies, then at least the Aragonese crown will pass to a Trastámara king. And lo and behold, on May 3, 1509 Germaine de Foix gives birth to Joan, Prince of Girona. However, Joan only survives for a few hours. Despite trying for the next seven years, Joan would be the only child between Ferdinand and Germaine. In 1516, King Ferdinand of Aragon died, passing the crown of Aragon to Charles von Habsburg, his grandson and the eldest son of Juana la Loca and Phillip of Habsburg. Charles would go on to rule Aragon and Castile, as well as the duchy of Burgundy and the Holy Roman Empire from 1516 to his death in 1556 as one of the most powerful monarchs in Europe.
If it weren’t for these extremely opportune series of events, the Habsburg dynasty would have never inherited the thrones of Castile and Aragon, and the Holy Roman Empire would not be able to use the Spanish Empire’s wealth in the New World to fund its endeavors. And the Spanish Empire would have had its enormous wealth without spending it lavishly funding wars in central Europe and fighting the Protestant Reformation. If any one of Isabella and Ferdinand’s children besides Juan had been born male, or if one of the heirs that died survived, or if Germaine de Foix had given Ferdinand another son, the history of the 16th century would be very different.
And what of the Maria and Catherine, the two younger sisters of Juana la Loca? Maria would marry Manuel I of Portugal after the Princess Isabella’s death, and go on to bear eight children. Catherine, meanwhile, went on to play her own part in another succession crisis, as the first wife of Henry VIII and the mother of Queen Mary I.