Based on the recent historical and archaeological studies of the early states in irrigation societies, I demonstrate the hypothesis that the states were brought into being by the selfish motives of the chieftains of irrigation societies for securing the better terms of trade with external societies providing new necessary goods vital to survival of those societies. The logic of the hypothesis is proved in the analytical framework of a two-stage bargaining game and that of a hierarchical cooperative game. The main results of the analysis are corroborated by some historical evidences.