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How did Philip the Handsome's marriage to Joan of Castile expand Habsburg power?

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Stone ruins of Habsburg Castle, also known as Habichtsburg, stand on a hill above green vineyards in Aargau canton, Switzerland, with a flag flying atop the tower. — House of Habsburg Ruins of the castle of Habsburg, or Habichtsburg (“Hawk's Castle”), built 1020, the original seat of the house of Habsburg, Aargau canton, Switzerland.

The marriage of Philip the Handsome to Joan of Castile in 1496 was a pivotal event that expanded the influence of the Habsburg dynasty into the Iberian Peninsula.1

By marrying Joan, the daughter of Ferdinand II of Aragon and Isabella I of Castile, Philip secured a claim to the Spanish thrones. Following the death of Isabella in 1504, Joan became the heiress of Castile, and Philip was recognized as king consort. Although Philip died shortly after in 1506, his marriage ensured that the Spanish succession would pass to his son, Charles I of Spain (also known as the Holy Roman Emperor Charles V).12

When Charles ascended to the throne in 1516, he united the crowns of Castile and Aragon under his rule, firmly establishing the Habsburg dynasty in Spain for nearly two centuries. This union integrated the vast Spanish territories—including those in the New World—into the broader Habsburg domains, significantly consolidating the family's power across Europe.12