The answer to this can be found in Chapter 3. There, Diamond lists a number of proximate causes. These are found on page 80 in the paperback edition of the book. The proximate causes include:
- Military technology. Diamond includes guns, steel weapons, and horses among these.
- Infectious diseases that killed many Indians.
- Centralized political organization that was much more systematic and organized than that of the Incas.
According to Diamond, these proximate causes allowed the Spanish to conquer the Incas. He then devotes most of the rest of the book to understanding why the Spanish had these advantages and the Incas did not.