What role did europeans play in the decline of the aztec and inca civilizations?

The arrival of the European conquerors brought imbalance to the hierarchy of the Aztec and Incan Empires.

According to the Aztec prophecy, The Aztec god Quetzalcoatl would one day return in the form of a light skinned human heralding from the east. When Hernan Cortes arrived, Emperor Moctezuma mistook him for Quetzalcoatl. With the supreme ruler of the natives bowing to Cortes, it didn't take time for Spain to swallow the Aztec empire.

The downfall of the Inca was caused by the invasion of Francisco Pizarro and his men. Emperor Atahualpa just recently won a war against his brother who was equally vying for the throne and his army was greatly weakened. Pizarro took this opportunity to lead Atahualpa into a trap and successfully detained him. He made the emperor embrace Christianity before killing him.