The document provides an overview of a class lecture on the Peloponnesian War between Athens and Sparta in ancient Greece. It summarizes key events leading up to the war, including rising tensions between the two city-states and their respective alliances as Athens' power and influence grew. The war lasted from 431 to 404 BCE and resulted in Athens surrendering and losing its empire, leaving all of Greece poorer. Realist theories are discussed as explanations for the causes of the war, focusing on the security dilemma as Athens and Sparta engaged in an arms race out of fear and insecurity regarding each other's growing military power.