Why were the Aztecs and Incas so easily defeated by smaller Spanish forces?

A civil war

Explanation:The Spanish explorer Francisco Pizarro invaded the Incan Empire in 1532, seeking riches. The Inca had already had some contact with Europeans, and many had died of European diseases. The empire was also weakened by a civil war between two ruling brothers. Pizarro manipulated the two sides, eventually defeating both.

The Spanish forces although smaller had guns armor and horses while the native people had primitive spears and bows.

History is not an exact science. The facts remain the same for all historians to people but their viewpoints always differ by definition, leading to different interpretations.

So, coming into an agreement on historical research is really, really hard, but then again - human society is built on a variety of opinions.