His successor and brother Cuitláhuac took control of the Aztec empire, but was among the first to fall from the first smallpox epidemic in the area a short time later. Unintentionally introduced by Spanish conquerors, among whom smallpox, measles, and other contagious diseases were endemic, epidemics of Old World infectious diseases ravaged Mesoamerica starting in the 1520s. The exact number of deaths is disputed, but unquestionably more than 3 million natives who they had no immunity. Other sources, however, mentioned that the death toll of the Aztecs might have reached 15 million (out of a population of less than 30 million) although such a high number conflicts with the 350,000 Aztecs who ruled an empire of 5 million or 10 million. Severely weakened, the Aztec empire was easily defeated by Cortés and his forces on his second return with the help of state of Tlaxcala whose population estimate was 300,000. The native population declined 80–90% by 1600 to 1–2.5 million. Any population estimate of pre-Columbian Mexico is bound to be a guess but 8–12 million is often suggested for the area encompassed by the modern nation. The territory became part of the Spanish Empire under the name of New Spain in 1535. Mexico City was systematically rebuilt by Cortés following the Fall of Tenochtitlan in 1521. Much of the identity, traditions and architecture of Mexico developed during the 300-year colonial period from 1521 to independence in 1821. The 1521 capture Tenochtitlan and immediate founding of the Spanish capital Mexico City on its ruins was the beginning of a 300-year-long colonial era during which Mexico was known as "Nueva España" (New Spain). The Kingdom of New Spain was created from the remnants of the Aztec empire. The two pillars of Spanish rule were the State and the Roman Catholic Church, both under the authority of the Spanish crown. In 1493 the pope had granted sweeping powers to the Spanish crown, with the proviso that the crown spread Christianity in its new realms. In 1524, King Charles I created the Council of the Indies based in Spain to oversee State power its overseas territories; in New Spain the crown established a high court in Mexico City, the "Real Audiencia," and then in 1535 created the viceroyalty. The viceroy was highest official of the State. In the religious sphere, the diocese of Mexico was created in 1530 and elevated to the Archdiocese of Mexico in 1546, with the archbishop as the head of the ecclesiastical hierarchy, overseeing Roman Catholic clergy.