How were the Aztecs defeated?

the spanish were already inside tenochitlan, and so when they attacked the Aztecs they were driven back and driven out of the city, but the spanish left European diseases that the Aztec's immune system wasn't immune to, causing them to begin to die and weakened them, so that when cortez returned to tenochitlan, with all of the Aztecs is enemies. they were to weak to last out the entire seige