Tada!! There's no point in just destroying a place completely. You need to occupy to derive any benefit (wealth, oil, slaves etc) from it. You cannot kill everyone, so it is possible for you to subjugate them to work for you. Thus, if you cannot kill everyone, they can possibly kill you. That's why the govt having superior firepower is pointless against its own armed citizenry. Also works for a conquering country against an armed populace. Do you think the aristocratic class would just want an empty country filled only with their soldiers?