This video explains that once the Spanish arrived in the Americas, epidemics of Smallpox ravaged the Incan societies, killing as many as 90% of them including the direct successors for the royal families, causing more obscure members to compete for the throne in vicious and costly civil wars that further ripped the empire apart.  The video also explains how the Conquistadors came to the Americas under the premise of “Christianizing” it, but they also came to become wealthy.