The Pope Innocent VII gave to Isabel of Castile and Ferdinand of Aragon the Catholic Monarchs dignity. Both Isabel and Fernando had to fight against other pretenders for the crown of Castile and Aragon. When she married, became kings of each of their kingdoms and kings, consorts of the other, establishing a union that was the backbone of modern Spain. The Catholic monarchs strove to strengthen the political and legal institutions of Castile, Aragon and Navarra, who had joined their domains. In 1492 completed the reconquest of Spain when they took the Moorish kingdom of Granada and promoted the discovery and colonization of America. When Elizabeth died in 1504, Fernando kept the crown of Aragon and exercised the regency of Castile, who inherited his daughter Joan. His grandson, Charles of Habsburg, son of Juana and Philip the Fair, was the first king of united Spain as Charles I, when Fernando died in 1516. Consummate political skill and military chief, Fernando was Machiavelli who inspired his work The Prince.