The Peloponnesian War was an ancient Greek military confrontation between the Spartan-led Peloponnesian League and the Athenian-led Delian League. The war lasted for over 27 years, from 431 to 404 BCE and resulted in the defeat of Athens by Sparta. The war had a profound impact on the Greek world, transforming the balance of power and changing the geopolitical landscape of the Eastern Mediterranean. Thucydides chronicled the war in his famous book, "The History of the Peloponnesian War." The conflict was a complex one with multiple causes, but it is often seen as a struggle between two competing systems of government: democracy and oligarchy. However, it is important to note that this is a simplification and that there were many factors at play in the outbreak of the war.