Page "Maximilian I, Holy Roman Emperor" Paragraph 1

By marrying his son Philip the Handsome to the future Queen Joanna of Castile in 1498, Maximilian established the Habsburg dynasty in Spain and allowed his grandson Charles to hold the throne of both León-Castile and Aragon, thus making him the first de jure King of Spain.

Having outlived his father Philip, Charles succeeded Maximilian as Holy Roman Emperor in 1519, and thus ruled both the Holy Roman Empire and the Spanish Empire simultaneously.