This article shows that Thucydides is deliberately enigmatic about the causal effects of his speeches, and that this enigma is central to understanding his account of the outbreak of the Peloponnesian War. First, it shows that Thucydides was fascinated by the way speeches might not only illuminate events but also contribute to causing them. This ambiguous aspect of the speeches has been underappreciated in recent scholarship, but it is a theme to which the historian frequently returns. Second, the article shows that Thucydides explores this ambiguity in particular detail through the debates in Book 1. Using echoes of the “truest explanation” from 1.23.6 in the speeches, Thucydides suggests that the war might have started through a process of self-fulfilling prophecy, rather than—as some recent scholarship has suggested—self-fulfilling perception. Finally, this article suggests a new reading of 1.23.6, which reflects the ambiguous nature of the speeches in the ambiguous term πρόφασις, again hinting at the possibility of a self-fulfilling prophecy. This article therefore challenges not only the idea of the “Thucydides Trap,” which continues to dominate conceptions of Thucydides among political scientists, but also the way all readers of his work approach both Book 1 and the role of the speeches in general.