The answer to this is closely related to what I said in my answer to your question about kleptocracy. Governments are religions are important agents of conquest because they allow a society to put together the kinds of military forces that they need to conquer others.
When people live in small groups with loose leadership, they cannot put together huge forces to conquer other people. Individuals may be willing to fight to defend their homes or to gain some honor, but they will rarely be willing to go out and conquer others in extended campaigns. Governments and religion make this possible.
Governments make it possible because they take wealth from the people and use it for their own ends. A government can take money and goods in taxes or tribute and use them to equip and pay large armies. They can force people to fight in those armies. They are helped in this, Diamond says, by religion. Religion makes people think that it is their moral duty to fight for their country. It equates the desires of the rulers with the desires of the gods. This makes it more likely that people will be willing to go out and fight.
Thus, religions and governments make it much more possible to create large armies that are willing and able to go out and conquer.