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In his review of international relation's history, George
Modelski has emphasized the lack of a centralized power in the world system. Discussing the role of the great powers in the

formation of the world system he has taken into account economic and political factors. Modelski suggests that the history

of the world could be broken into five periods each characterized by the superiority of a world power. He believes that the nation ¬
state has been the central entity in the formation of the world system.