The Spanish explorer Francisco Pizarro invaded the Incan Empire in 1532. The Inca had contact with Europeans and many died of European diseases. The empire was also weakened by the civil war between two brothers who wanted to rule after the death of the King. Pizarro used the weakness to his advantage; he manipulated the two brothers and eventually defeated both sides.
Aztec was the first empire to fall. Several of the local tribes joined Hernan Cortes in his campaign against the Aztecs in 1521. Small Pox was also a factor causing a major plague; it killed thousands including the Aztec nobility. It is difficult for a society to function when its people are dead or dying. This made it easy for the Incan and Aztec Empires to be defeated.