Which factor contributed to the fall of both the Incan and the Aztec empires? Civil wars continued.
Their emperors betrayed them.
Their enemies had larger military forces.
New diseases were introduced.

New diseases were introduced. When Chris Columbus and other Spanish explorers found the Americas, they brought diseases such as smallpox, measles, and the plague to the regions. The people living in Afro-Eurasia gained immunity to such diseases, unlike the Incans and Aztecs. Thus, their population diminished by 90%, resulting into the fall of their empires.