What were the three reasons why the Spanish defeated the Aztecs?

1) Montezuma invited the Spanish into the city, believing his people would better be able to defend the city if the enemy were withing their reach.

2) The Spanish used a tactic Monntezuma did not expect.... knowing they were outnumbered and unfamiliar with the location in which they found themselves their first act of war was to snatch the emperor and imprison him, leaving the Aztec peple without their leader.

3) Though the Spanish were well prepared for their excursion and subsequent conflict, one of the most valuable weapons they brought into the new world was disease. The Aztec people had never been exposed to many of the illnesses the Spanish brought with them from their homeland, and as a result, many of the Aztec people died.