Diamond's main idea in Chapter 14 is that agriculture leads to larger and more complex societies and that those societies have advantages over smaller and less complex societies.
Diamond argues that agriculture allows more people to live together in denser communities. He then says that these dense communities need stronger governments to prevent conflict among these large populations of unrelated people. These governments then become more complex and more organized and take more of the resources of the society for their own use.
Such large and complex societies are, to Diamond, inherently more powerful than smaller ones. They have governmental institutions that can take resources from the people and use them to build things like armies. They have religions that can inspire people to fight and to die for the state. The fact that they have these things makes them formidable opponents for any less organized society.
In Chapter 14, then, Diamond shows how agriculture leads to more complex societies and why those societies are more powerful than smaller societies.