Our main source for the history of the Peloponnesian War is the Athenian author Thucydides, but he sometimes makes mistakes. One of these is a blind eye for Persian influence on the war. As a consequence, he underestimated the importance of the revolt of one Amorges.
This man was the bastard son of Pissuthnes, the satrap of Lydia, who had revolted against the Achaemenid king Darius II Nothus in 420. Although the great king's representative Tissaphernes had arrested Pissuthnes, Amorges continued the struggle from the Carian city Iasus, and received naval support from Athens. It is probable that as a consequence, Darius decided to side with Sparta, and made the fall of Athens inevitable.
This story cannot be found in Thucydides' History of the Peloponnesian War. Still, we can read between the lines, and we know what we must be looking for because it is hinted at by Andocides, a Greek politician whose speeches were already regarded as "classics" in the third century BCE. In 395, war between Athens and Sparta was resumed, but in 392/391, it seemed as if an armistice and a treaty were possible. Andocides supported the Spartan peace offer and delivered his speech On the Peace with the Spartans. He warned the Athenians against a mistake he presents as common: to support the weaker side in a conflict and incurring the wrath of a powerful enemy. One of the examples he offers, is Amorges.
Obviously, the people he was addressing shared Andocides' assessment of the support their fathers had given to the rebel: otherwise, this example would have been counterproductive. We will probably never know what made Darius support Sparta, but we do know that among the Athenians who had survived the war, it was widely believed that it had been their support for Amorges.
The translation of §28-32 of On the Peace with the Spartans was made by K. J. Maidment.
Andocides on Amorges
 We have a choice between two alternatives, that of joining Argos in fighting Sparta, and that of joining Boeotia in making common peace with her.
 Now what alarms me above all else, gentlemen, is our old, old fault of invariably abandoning powerful friends in preference for weak, and of going to war for the sake of others when, as far as we ourselves are concerned, we could perfectly well remain at peace.
 Thus -and it is only by calling the past to mind that one can properly determine policy- we began by making a truce with the Great King and establishing a permanent accord with him, thanks to the diplomacy of my mother's brother, Epilycus, the son of Teisander.
 But later the king's runaway slave, Amorges, induced us to discard the powerful support of his master as worthless. We chose instead what we imagined to be a more advantageous understanding with Amorges himself.
 The king in his anger replied by allying himself with Sparta, and furnished her with five thousand talents with which to prosecute the war; nor was he satisfied until he had overthrown our empire. That is one instance of such policy.