This thesis examines the historiographical treatment of interstate relations in Thucydides' History of the Peloponnesian War and seeks to apply this treatment of diplomatic rhetoric to a historical understanding of fifth-century Greek perceptions of their inter-poleis condition. I analyze three sets of speeches from the first, third, and sixth books of Thucydides' History of the Peloponnesian War: the speeches of the Corcyraeans and the Corinthians at Athens, the Plataean Debate, and the speeches of Nicias and Alcibiades to the Athenian Assembly on the proposed Sicilian Expedition. After investigating a passage from the preface, in order to demonstrate how Thucydides views his speeches as both crucial and unclear when attempting to understand the War's αἴτιαι (1.23.6), I examine the speeches in light of how Thucydidean rhetorical strategies highlight Athenian and Spartan rejection of moral aspects of negotiation of interstate relations, namely the values of justice and reciprocity, balancing power, and restraint. By subverting assumptions about how such older, established methods of diplomacy between poleis have historically functioned successfully, Thucydides is able to drive forward his narrative about the death of interstate relations during the War. Finally, I attempt to show how the speeches are designed to influence the ideologies of Thucydides' readers, his fellow Greek contemporaries, about warfare in a negative way—regarding the inefficacy of pursuing positive interstate relations and the general futility of striving for peace.