In 1516, Ferdinand II of Aragon, grandfather of the future Holy Roman Emperor Charles V, died. Due to a combination of (1) the traditions of dynastic succession in Aragon, which permitted maternal inheritance with no precedence for female rule; (2) the insanity of Charles's mother, Joanna of Castile; and (3) the insistence by his remaining grandfather, Maximilian I, that he take up his royal titles, Charles initiated his reign in Castile and Aragon, a union which evolved into Spain, in conjunction with his mother. This ensured for the first time that all the realms of the Iberian peninsula (save for Portugal) would be united by one monarch under one nascent Spanish crown, with the founding territories retaining their separate governance codes and laws. In 1519, already reigning as Carlos I in Spain, Charles took up the imperial title as Karl V. The balance (and imbalance) between these separate inheritances would be defining elements of his reign, and would ensure that personal union between the Spanish and German crowns would be short-lived. The latter would end up going to a more junior branch of the Habsburgs in the person of Charles's brother Ferdinand, while the senior branch continued rule in Spain and in the Burgundian inheritance in the person of Charles's son, Philip II of Spain. In addition to conflicts between his Spanish and German inheritances, conflicts of religion would be another source of tension during the reign of Charles V. Before Charles even began his reign in the Holy Roman Empire, in 1517, Martin Luther initiated what would later be known as the Reformation. At this time, many local dukes saw it as a chance to oppose the hegemony of Emperor Charles V. The empire then became fatally divided along religious lines, with the north, the east, and many of the major citiesóStrasbourg, Frankfurt and Nurembergóbecoming Protestant while the southern and western regions largely remained Catholic.