In the Peloponnesian War, the Greeks fought against each other. The war was primarily fought between two major Greek city-states: Athens and Sparta. Athens led the Delian League, an alliance of various Greek city-states, while Sparta led the Peloponnesian League, which included many other city-states in the Peloponnese region. The conflict was essentially a civil war among the Greek city-states, with Athens and Sparta serving as the primary belligerents.