The answer to this question can be found in Chapter 14. There, Diamond discusses the different kinds of social or political organization into which human societies can fit.
The four types of societies are bands, tribes, chiefdoms, and states. These are listed in order from the least complex and smallest to the biggest and most complex. Bands are nomadic groups that are closely related to one another and number only in the dozens. Tribes live in one village in clan groupings and number in the hundreds. Chiefdoms are made up of thousands of people in one or more villages. Diamond says that states have at least 50,000 people in villages and cities. You can refer to Table 14.1 for more details on these types of society.
On p. 281 in the paperback edition of the book, you can find Diamond’s explanation of why states are better than other societies in war and conquest. He gives the following reasons. First, states simply have more people. Second, they also tend to have better technology since they have larger populations with more possible inventors. Third, they have centralized governments. This allows them to have one decision maker. This gives them “the advantage at concentrating troops and resources” when compared to other, more egalitarian societies. Finally, the “official religions and patriotic fervor of many states make their troops willing to fight suicidally.”
For these reasons, Diamond says that states tend to be more militarily powerful than bands, tribes, or chiefdoms.