Thucydides’ History of the Peloponnesian War is one of the most significant historical and political texts of Ancient Greece, enjoying a broad appeal among the educated general public since at least 1628. The past decade has seen the historian garner significant attention even in the popular press, as scholars and politicians alike have sought to employ the History to analyze current international relations. Despite this popularity, the complexity of Thucydides’ Greek has left the original language surprisingly challenging. Commentary on Thucydides, Book 3 remedies this situation by offering detailed linguistic explanations and grammatical clarifications designed to appeal both to seasoned Classicists and to a broader group of non-specialist readers who may still be developing their Greek language skills. Starting with 428 BCE, Book 3 covers a critical period of the Peloponnesian War in which the conflict began to manifest its extraordinary violence and scale. The book contains influential and controversial discussions, including Thucydides’ own analysis of the nature of war and the ways that it teaches “lessons of violence” to individuals and states. Book 3 also features the famous Mytilenean Debate, an argument premised on the thesis that all international relations are, or should be, fundamentally amoral. Educated readers have always looked to Thucydides in turbulent times, and this commentary will open up his text to a wider audience.