Smallpox killed hundreds of thousands of the Native Americans, which weakened the Aztec Empire and created political and social instability. Second, Hernan Cortes and his men had superior weaponry. The Spanish had steel weapons, cannons, and horses; these advantages helped them defeat the much larger Aztec army.
The Spanish and their Indian allies outnumbered the Aztecs which led to their defeat. Disease was a very important factor that led to the fall of the Aztec Empire. The Spanish brought many deadly diseases when they arrived in Mexico. The diseases, like smallpox, caused many Aztec die
Hope this helped,
-Anime
