The answer to this can be found in Chapter 3. There, Diamond lays out a number of factors that allowed Spain to conquer the New World rather than having Native Americans conquer Spain. They include:
- Military technology. The Europeans had guns and steel weapons.
- Infectious diseases. These "germs" killed many Native Americans, helping to weaken their societies.
- Ships and writing. These were two more things that the Europeans had and the Americans didn't.
- Centralized political organization. This allowed the Spanish to be more organized and disciplined than the natives.
All of these were proximate causes of Europe's ability to conquer. The ultimate cause of this, however, was the fact that agriculture arose in Eurasia long before it did in the Americas, thus allowing Europe to become stronger than the New World.