There are three main reasons why the Spanish were able to conquer and colonize the Americas:
Disease. One of the biggest hurdles to Spanish victory in the New World was population—some conquistadors only brought a handful of men and were up against empires that were a million or more strong. For examples, Hernan Cortes brought 500 men into battle against the Aztec Empire and won. One of the major reasons why he won was became of disease transmission. The native inhabitants of the Americas had not experienced smallpox and, with the Columbian Exchange, diseases from the Old World were introduced to people without any immunity. Smallpox ravaged the populations of the New World and anywhere between 50 and 80% of the total population died. While conquistadors like Hernan Cortes in Mexico and Francisco Pizarro had great weapons and strategies, their greatest ally was disease.
Better weaponry. Iron weapons and technology did not exist in the Americas. Gunpowder was a Chinese creation that, by the time of the Age of Exploration, had spread throughout Europe. The Spanish possessed better weapons and gunpowder. The indigenous Americans, although fearsome warriors, were no match for guns.Internal
Struggles. In the Inca Empire, prior to Spanish arrival, a Civil War ravaged the country. Atahualpa and his older brother Huascar fought each other for control of the Inca Empire. Atahualpa eventually won, and the empire he inherited was filled with division and weakened from war. Enter Francisco Pizarro. Taking advantage of the weakened state, disease, and superior weaponry and strategy, Pizarro was able to conquer the Incans. In the Aztec Empire, Hernan Cortes used some of the Aztec's enemies in the Yucatan and took advantage of the internal strife between Mesoamerican groups.
