This dissertation reinterprets Thucydides’ perspective on democracy and the possibilities of wise democratic decision-making by examining moments of public deliberation in his account of the Peloponnesian War. It finds that for Thucydides, poor democratic decisions stem from four fallacious patterns of thought and behavior. Firstly, the Athenians misunderstand the relation between their leaders and themselves. Rather than ruling critically, they place all their trust in Pericles. The demagoguery and deliberative dysfunction that follows Pericles’ death are his legacy, and show us not that democracy needs monarchical leaders, but that such leaders distort and weaken democracy over the long term. Secondly, the Athenians display a hubristic confidence in their powers of reason, which leads them to think that they can guarantee success in war through commensurate planning. Thucydides’ narrative focuses on the power of chance to upend the best-laid plans, and shows how Athenian confidence contributes to their inability to understand and speak to other cities. Thirdly, the Athenian belief that their actions should reflect their character, while an integral part of their decision-making process, can encourage recklessness and foreclose beneficial change when harnessed unimaginatively. This fallacy is prominent in Alcibiades’ exhortation to invade Sicily, while Nicias’ failure to stop the invasion stems from a failure to speak to the Athenian character and open up alternative paths for its expression and development. Fourthly, the Athenians seem particularly susceptible to—though at their best moments reject—a rhetoric that places partisanship over patriotism. Throughout Thucydides’ narrative, unscrupulous speakers deploy partisan slogans to discredit opposition and undermine constructive debate. In the final book of Thucydides, however, the Athenians set aside partisanship and its slogans and embrace the compromise government of the Five Thousand to save the city from destruction. I propose that Thucydides did not intend a blanket condemnation of democracy as a form of government, or of the people as an irrational mob ineluctably bound for disaster. He is not the democratic fatalist that he is often taken to be. Rather, he demonstrates how a people may be misled by a complex interplay of beliefs and circumstance, and leaves open the possibility that, absent these specific fallacious modes of reasoning, democracy can avoid the disaster that befell Athens.