How a small number of Spaniards were able to bring down the Aztec and the Inca empires?

The Spaniards has all ready learn how to use metals as weapons in the use of swords,they also had knowledge of gun powder. Until the arrival of the Spaniards the Aztecs had never seen a horse before which the Spaniards were very skillful in the use of using this very large and strong and fast beast in war. But the number one weapon the Spaniard possess was disease that ran rampage thur the Aztec population.