In this lecture, Professor Kagan describes the events that lead up the Peloponnesian War. He argues that the rise of Athenian power and the concomitant challenge to Spartan dominance pointed to potential conflict. However, Professor Kagan also points out that there were many people who did not want war and that therefore war was not inevitable. The Thirty Years Peace was negotiated, and Professor Kagan finally argues that its clause for arbitration was the key clause that could have prevented war.