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Very interesting, I read something very similar in Trautmann's Arthashastra - ancient India had republics and kingdoms. Kingdoms were more diverse and had greater wealth whereas republics were often created by a single group of people and were smaller. This meant that kingdoms could get into an expansionist mode more often and easily than republics with the result that many of the republics died out. From what I understood a republic in that time was a kind of oligarchy.