Peloponnesian War
The Athenian-led Delian League and the Spartan-led Peloponnesian League fought for twenty-seven years until Sparta won; the world of the Greek poleis declined in its wake.
Friedrich Wilhelm Putzger (1849-1913). · Public domain · Source
As Athenian power grew, the Peloponnesian League around Sparta stood against it. When war broke out in 431 BC Pericles brought the citizens inside the walls and fought at sea, but the next year a plague swept the crowded city, killing many, and Pericles himself died of illness in 429 BC.
Athenian politics was then shaken by demagogues who relied on popular favour, and the Sicilian Expedition of 415 BC ended in disaster. Sparta, building a navy with Persian money, destroyed the Athenian fleet at Aegospotami in 405 BC, and Athens surrendered the following year.
Thucydides recorded the war in his History of the Peloponnesian War. Afterwards Sparta and Thebes in turn fought for supremacy while the poleis wore themselves out, and in 338 BC they bowed to Macedon in the north.