This is an exploration of the nature and implications of great power rivalry which brings together historians, classicists and political scientists to examine the similarities and differences between present-day superpower relations and the period preceding the outbreak of the Peloponnesian War in 431 BC. It uses these two conflicts to test the validity and explanatory power of realism, power transition theory and other approaches to undestanding war, and seeks to distinguish between generic and idiosyncratic causes and manifestations of conflict.