left Milan under French control and recognized Spanish hegemony in northern Navarre. Ferdinand would die later that year. Ferdinand's death led to the ascension of young Charles to the throne as Charles I of Castile and Aragon, effectively founding the monarchy of Spain. His Spanish inheritance included all the Spanish possessions in the New World and around the Mediterranean. Upon the death of his Habsburg father in 1506, Charles had inherited the Netherlands and Franche-Comté, growing up in Flanders. In 1519, with the death of his paternal grandfather Maximilian I, Charles inherited the Habsburg territories in Germany, and was duly elected as Holy Roman Emperor that year. His mother Joanna remained titular queen of Castile until her death in 1555, but due to her mental health and worries of her being proposed as an alternative monarch by opposition (as happened in the Revolt of the Comuneros), Charles kept her imprisoned. At that point, Emperor and King Charles was the most powerful man in Christendom. The accumulation of so much power by one man and one dynasty greatly concerned Francis I of France, who found himself surrounded by Habsburg territories. In 1521 Francis invaded the Spanish possessions in Italy and Navarre, which inaugurated a second round of Franco-Spanish conflict. The war was a disaster for France, which suffered defeats at Biccoca (1522), Pavia (1525, at which Francis was captured), and Landriano (1529) before Francis relented and abandoned Milan to Spain once more. Spain's overseas possessions in the New World were based in the Caribbean and the Spanish Main and consisted of a rapidly decreasing indigenous population, few resources of value to the crown, and a sparse Spanish settler population. The situation changed dramatically with the expedition of Hernán Cortés, who, with alliances with city-states hostile to the Aztecs and thousands of indigenous Mexican warriors, conquered the Aztec Empire in 1521. Following the pattern established in Spain during the Reconquista and in the Caribbean, the first European settlements in the Americas, conquerors divided up the indigenous population in private holdings encomiendas and exploited their labor. With Americas colonization, Spain gave vast new indigenous populations to convert to Christianity and rule as vassals of the crown. Charles established the Council of the Indies in 1524 to oversee all of Castile's overseas possessions. Charles appointed a viceroy in Mexico in 1535, capping the royal governance of the high court, Real Audiencia, and treasury officials