In this celebrated sentence Thucydides gives his opinion more specifically than anywhere in the narrative of Books 6 and 7 on the reasons for the failure of the Sicilian expedition. Unfortunately, however, he expresses his views with some lack of clarity, and this has led to disagreement among modern scholars who have sought to determine precisely what he means. It is also unfortunate that he does not state the evidence upon which he bases his conclusion that the expedition failed because of struggles for political leadership at Athens. The main purpose of this paper is to consider the meaning and implications of this sentence, which can, I believe, be clarified by reference to other passages in which Thucydides uses when debating the causes of past actions. A closely allied problem, that of the relationship between this sentence and the narrative of Books 6 and 7, will also be discussed. A. W. Gomme has maintained that this relationship throws some light upon the date of composition of Books 6 and 7. There is no doubt whatever that the sentence was written at or after the end of the Peloponnesian war (ibid. 12), and if there are differences of outlook, even inconsistencies, between it and the narrative which show that Thucydides cannot have written, or at any rate conceived, both at the same time, he almost certainly wrote the narrative before the end of the war.2 In my opinion there is inconsistency on one point only, but here the inconsistency is sufficiently marked to suggest not only that Books 6 and 7 were written considerably earlier than 2. 65. 11, but that they were written within the next few years after the conclusion of the Sicilian campaign.