1. Disease: The arrival of European explorers and settlers brought with them diseases such as smallpox, measles, and typhus, which decimated the indigenous population of the Americas. It is estimated that up to 80% of the Aztec population died as a result of these diseases.
2. Internal dissent: The Aztec Empire was not a monolithic entity, and there were many different groups and factions within it. Some of these groups were dissatisfied with the rule of the Aztec emperor, Montezuma II, and they were willing to ally with the Spanish in order to overthrow him.
3. Military technology: The Spanish had a significant advantage in military technology, including firearms, cannons, and steel armor. These weapons were far more effective than the weapons used by the Aztecs, and they allowed the Spanish to defeat the Aztec armies in a series of battles.
4. Leadership: Montezuma II was not a particularly effective leader, and he made several strategic mistakes that contributed to the fall of the Aztec Empire. For example, he initially allowed the Spanish to enter Tenochtitlan, the Aztec capital, without a fight. He also failed to take decisive action against the Spanish when they began to build their own ships and weapons.
5. Natural disasters: The Aztec Empire was also hit by a series of natural disasters, including droughts, floods, and earthquakes. These disasters weakened the empire's infrastructure and made it more vulnerable to attack.