How did Philip the Handsome’s marriage to Joan of Castile help bring Spain under Habsburg rule?

The marriage of Philip the Handsome to Joan of Castile in 1496 was a pivotal element of the Habsburg matrimonial strategy, which sought to expand the dynasty's power through strategic alliances rather than war.1

Philip was the son of Holy Roman Emperor Maximilian I and Mary of Burgundy. Maximilian arranged for Philip to marry Joan (the Mad), the daughter of Ferdinand II and Isabella I. Through the unexpected deaths of other heirs in the Spanish royal line, Joan became the heiress to the crowns of Castile and Aragon.23

Following the death of Isabella in 1504, Philip claimed the Castilian throne through his wife. Although Philip died prematurely in 1506, his son Charles V inherited these vast Spanish dominions—including territories in Italy and the Americas—upon the death of Ferdinand in 1516. This succession firmly established the Habsburg dynasty in Spain for nearly two centuries.21