1. European diseases: The introduction of European diseases, such as smallpox, measles, and influenza, decimated the native population. It is estimated that up to 90% of the Aztec population died from disease within a few years of the arrival of the Spanish.
2. Spanish military technology: The Spanish had a significant advantage in military technology, including guns, cannons, and steel armor. The Aztecs were unable to match the firepower of the Spanish, and their traditional weapons, such as obsidian-tipped spears and clubs, were no match for the Spanish weaponry.
3. Political divisions: The Aztec Empire was divided into several different city-states, which were often at war with each other. The Spanish were able to exploit these divisions and ally themselves with some of the native groups against the Aztecs.
4. Montezuma's leadership: The Aztec emperor, Montezuma, made a number of strategic errors that contributed to the fall of the empire. He initially welcomed the Spanish and allowed them to enter the capital city, Tenochtitlan, without a fight. He also failed to take decisive action against the Spanish when they began to attack the city.
5. Spanish strategy: The Spanish, led by Hernán Cortés, were able to use their superior technology and alliances with native groups to gain a foothold in Mexico. They then laid siege to Tenochtitlan and eventually captured the city in 1521.
The fall of the Aztec Empire marked the beginning of Spanish rule in Mexico, which lasted for over 300 years.