 Joanna inherited these territories in 1516 in a condition of mental illness.Charles, therefore, claimed the crowns for himself "jure matris", thus becoming co-monarch of Joanna with the title of "Charles I of Castile and Aragon" or "Charles I of Spain".Castile and Aragon together formed the largest of Charles's personal possessions, and they also provided a great number of generals and tercios (the formidable Spanish infantry of the time).