The cause of great power war is a perennial issue for the student of politics. Some 2,400 years ago, in his monumental History of the Peloponnesian War, Thucydides wrote that it was the growth of Athenian power and the fear that this power inspired in Sparta which rendered the Peloponnesian War somehow necessary, inevitable, or compulsory. In this new political psychological study of Thucydides' first book, S.N. Jaffe shows how the History's account of the outbreak of the war ultimately points toward the opposing characters of the Athenian and Spartan regimes, disclosing a Thucydidean preoccupation with the interplay between nature and convention. Jaffe explores how the character of the contest between Athens and Sparta, or how the outbreak of a particular war, can reveal Thucydides' account of the recurring human causes of war and peace. The political thought of Thucydides proves bound up with his distinctive understanding of the interrelationship of particular events and more universal themes.