The main cause of the Peloponnesian War, fought between 431 and 404 BCE, was the power struggle between two major Greek city-states: Athens and Sparta. The conflict arose from the tensions following the Persian Wars when Athens emerged as a dominant naval power and established the Delian League, an alliance of city-states. Sparta, leading the Peloponnesian League, viewed Athenian expansion as an existential threat to its own power and the traditional oligarchic order. The war can also be attributed to various disputes and events, including the Sicilian Expedition. Its resolution resulted in the decline of Athenian power and significant changes in the Greek political landscape.