The Peloponnesian Wars were fought between Athens and Sparta, two city states in ancient Greece. The cause of the war was after the Athenians took control of the Delian League, Athens began to abuse their power. Spartans became angry and started to revolt against them. The war lasted about 25 years, but in the end Sparta was victorious. At the end of the war, Sparta became the new head of the Delian League.