Settled farmers would build irrigation systems supporting prosperous villages and towns. Eventually some tough guy, some well-organized priest, or some alliance of the two would bring a number of these urban centers under the rule of a single power, thereby forging a larger political unit—a confederation, a kingdom, an empire. Then a tribe of hardy nomads would come along, conquer the monarch of the moment, seize all his holdings, and in the process expand their empire. Eventually the hardy nomads would become soft, luxury-loving city dwellers, exactly the sort of people they had conquered, at
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