Final answer:
The Aztec empire fell due to a combination of Spanish military tactics, steel weapons, horses, and alliances with indigenous tribes. Disease outbreaks like smallpox played a crucial role as well. Moctezuma's belief that Cortes was a god did not contribute directly to the downfall of the empire.
Step-by-step explanation:
The downfall of the Aztec empire involved multiple factors, including military tactics, alliances, and diseases introduced by the Europeans. The Spanish conquest was not due simply to their superior numbers; in fact, Cortés's forces were greatly outnumbered by the Aztecs. The advantage came from steel weapons and armor, horses, as well as alliances with tribes subjugated by the Aztecs. A pivotal role in the fall of the Aztec empire was played by diseases like smallpox, introduced inadvertently by the Europeans, which decimated the indigenous population and significantly weakened their ability to resist the Spanish.
Regarding the options provided in the question, Option B: "Moctezuma thought Cortes was a god and welcomed him," does not directly contribute to the downfall. Although this belief may have affected initial interactions, the welcome did not contribute to the fundamental tactics and occurrences that led to the empire's demise. Instead, it was the combination of diseases, military superiority, and alliances that facilitated the collapse of the Aztec society.