I would add that this seems to have been the source of Roman power. It's codified into the Roman creation myth: Romulus fought the Sabines, and made them citizens in the same day, legend has it. Other city states demanded tribute. Rome demanded soldiers, and gave them a cut of the spoils. In that sense it was self propagating. I would actually draw a parallel to why America became what it was. People can become American much the same way that people could become Roman. The republic propagates because its increasing subjects have a stake in its propagation. Moreover, I would say that rampaging murder is a pretty stupid way to go about building a civilization, even if you're not going to grant citizenship to those you conquer. Is the goal to conquer ash?