The Athenian Empire came to an end when they were defeated by the Spartans in the Peloponnesian War, which lasted from 431-404 B.C. Below is a timeline of that war.
1st Stage of the Peloponnesian War from 431-421
Athens (under Pericles and then Nicias) successful until 424. Athens makes attacks the Peloponnese by sea and Sparta destroys areas in the countryside of Attica. Athens makes a disastrous expedition into Boeotia. They try to recover Amphipolis (422), unsuccessfully. Athens fears more of her allies would desert, so she signs a treaty (Peace of Nicias) that allows her to keep face, basically setting things back to how they were before the war except for Plataea and Thracian towns.
431 - Peloponnesian War begins. Siege of Potidaea.
430 - Plague in Athens.
429 - Pericles dies. Siege of Plataea.
428 - Revolt of Mitylene.
427 - Athenian Expedition to Sicily.
421 - Peace of Nicias.