After the Peloponnesian War, the Spartans set up an oligarchy in Athens, which was called the Thirty. It was short-lived, and democracy was restored. And due to an ill-conceived Spartan foreign policy, Athens was able to recover.
For Athens, there was a nasty epilogue. The regime of the Thirty was (text) even more oligarchic than the Four Hundred had been and was never popular, as even the Spartans recognized. Worse, the Thirty alienated Sparta's friends. The Thebans, who had asked for the sack of Athens and the killing of all its inhabitants during the peace negotiations, grew suspicious of the Spartan occupation of Athens, and started to support the democrats under Thrasybulus, who occupied Phyle, a fortress on the border of Attica and Boeotia.
The Thirty sent an army, but failed to achieve anything. They were divided, and tried to close their ranks. An even closer association with Sparta seemed the best way to remain in power, and Critias, whose loyalty to Sparta was not in doubt, became more influential. The moderate Theramenes was executed.
At the end of 404, the democrats suddenly seized Piraeus, which was easy: after all, the Long Walls had been destroyed. The democrats and oligarchs continued a civil war, which lasted until September 403, when the Spartan king Pausanias intervened and restored democracy (text). The oligarchs were given a free-conduct to Eleusis, but eventually reconciled themselves with the democrats.
Sparta did not long enjoy its victory. It owed much to prince Cyrus the Younger, who needed help when his father Darius II Nothus died in April 404 (at about the time of the capitulation of Athens) and was succeeded by Artaxerxes II Mnemon. The Spartan officer Clearchus, probably acting with tacit approval of his government, supported Cyrus when he revolted. Many Greek mercenaries, professional soldiers who had fought in the Peloponnesian War and were unable to settle, joined the expedition, which culminated in 401 in the battle of Cunaxa, in which Cyrus was killed.
After this, the Spartans interfered even more in the Persian zone of influence. King Agesilaus invaded the empire, and had considerable success. Now, the Persians started to support Athens, which rebuilt its Long walls (395). Next year, Conon, an Athenian admiral who had fallen into disfavor after the battle at the Aigospotamoi, returned with a large fleet. Athens had fully recovered
Or so it seemed. Of course, it owed its restoration to Persian money. The only victor in the Peloponnesian War was the great king.