In the 14th century a Muslim historian named Ibn Khaldun wrote about the pattern of history. Farmers would build irrigation systems supporting villages and towns. Later some warrior would bring these towns under his rule and form a united political entity, like a kingdom or an empire. Then a tribe of nomads would come along and conquer the kingdom, seize all the holdings and settle in their place and further expand the new empire. As time went by the nomads would assimilate and become soft city dwellers. Exactly the kind of people they had conquered and at this point another tribe of nomads would come along and conquer them and take their empire.