The Peloponnesian War was a three decade long conflict between two alliances of city states in Ancient Greece lasting from 431-400 BC. It was a total war, involving massive movements of armies and navies across land and sea; it affected the lives of almost every person in the Ancient Greek world, inflicting tremendous losses to life, property even, civilisation itself. The historian Donald Kagan in fact calls it a world war of the ancient era, as it drew in other powers from outside the Greek world. It drew in Persia, Macedonia, Sicily and even the smaller city states of Italy. The result: Ancient Greek civilisation which prior to the war was flourishing in a golden age was left devastated and ultimately conquered.
The Peloponnesian is studied till this day by scholars of international relations and geopolitics for the lessons it offers in terms of theory, and the parallels with various eras of world history, including our own.