The fall of the Aztec Empire to Spanish invaders was primarily due to a combination of disease, which significantly reduced the indigenous population, and the military might of Hernán Cortés. The Aztecs also faced internal strife and resentment from subjugated tribes. The Spanish not only had advanced weapons but also formed alliances with these tribes, which aided in their conquest. Additionally, the introduction of European diseases like smallpox devastated the Aztec population, weakening their societal structure and ability to defend against the invaders.