Cortes kidnapped the Aztecs' emperor in order to have them defenseless. Aztecs tried to fight back but were killed including their emperor.
3.Then, smallpox spread throughout Tenochtitlan. This made the Aztecs to become weaker so they weren't able to go against Cortes.
4.In June of 1521, Tenochtitlan destroyed by the Spanish.
Resources: (MEMT, p.473-474).