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 said to have dramatic unity in the sense that it is a single great action: as, by an analogous metaphor, the subject of Herodotus may be said to have epic unity, because the various parts, though they cannot be brought within the compass of one action, can be brought within the compass of one narrative. And, apart from this rudimentary dramatic unity, the Peloponnesian war has a further analogy to a drama in presenting a definite moment at which the cardinal situation is decisively reversed—as it is reversed in the Oedipus Tyrannis, for instance, when the king discovers that he is an incestuous parricide. That moment is the Sicilian expedition. The supreme test of "dramatic" quality in a history of the Peloponnesian war must be the power with which the historian has marked the significance of the Sicilian expedition as the tragic "revolution" (peripeteia), the climax of pity and terror, the decisive reversal. Thucydides has devoted the whole of his Sixth and Seventh Books to the events of those two years, thus at once marking the significance of the expedition as the turning-point of the war. And every reader knows with what tremendous effect he has traced its course, from the moment when the whole population of Athens was gathered at the