How were the Spanish able to conquer the Incas and Aztec empires?

Cortez was thought to be a returned emperor from the Aztec past that promised to come back one day and return to his thrown. With this belief the Aztecs didn't put up as much of a fight as they otherwise would have. Cortez also received support from other neighboring tribes.

Later Conquistadors defeated the Inca by tricking their emperor into a special banquet where they ambushed him. After the ambush the other Inca having seen their leader seized actually gave up the fight. They used the emperor by making him give orders to his people.