The fall of Athens

I wouldn't really say Sparta and Persia were allies.  Persia forced Sparta into agreement because Sparta had no choice.  None of the treaties between the two parties favored Sparta. They pretty much had to agree to submit to Persian authority.  The “liberators of Greece” propaganda that Sparta was using was no longer valid and I would imagine this affected Spartan morale, legitimacy, and influence.Athens lost because of failed alliances.  Argos would have been an important strategic ally to Athens, but after Kimon gained political influence, any chance of that alliance was lost and the Argives formed a (very) loose alliance with Sparta.

Seems like that was what I was taught.

I was asking because that has always been my understanding of why the Athenians lost.  The important point is not how many allies in total left, but that enough members of the Delian League abandoned athens because of the way Athens treated them that Sparta and its allies were able to defeat them.  Remeber that Sparta began the war with almost no navy.  They were almost exclusively a land power, but at the end of the war they could blockade Piraeus, that says something about both Sparta and Athens.  Sparta did not so much win the war as Athenian arrogance lost it.

Didn't Athens lose because their imperial pretensions and demands drove their allies out of the Delian League and into the arms of Sparta?

I think no, because not many left of their own accord, Athens just lost them. The neutral ones, Argos, Corinth, Thebes, are the ones who IMO really threw a wrench into both Athenian and Spartan ambitions.  Towards the end, Sparta was just as imperialistic as Athens was.  Athens pretentions weren't what lost the war for them, it was their fatal mistakes at Syracuse that did.  Athens original intent was not to invade Sicily but rather to protect their allies from the Syracusans. Their original plan did not involve enough troops or triremes to justify a full invasion.  Nicias was against any incursion from the beginning, (which Athens should have relived him from duty, but they did not) He said the Syracusans were stronger than they thought, so the Athenians decided to send more ships and troops...which is not what Nicias wanted...he wanted them to NOT go to Sicily. His speech backfired on him, however, and Athens changed plans from a small defensive campaign to an all out invasion.  Sparta and her allies viewed this as a violation of the Peace of Nicias, which it was.

It was a bunch of strategic miscalculations, combined with treachery and different opposing factions within Athens. Convicting the generals after the Battle of Arginusae was probably the stupidest thing Athens did. That left them with no experienced commanders. I'm only really arguing with you now because I just think that saying Athens lost because of imperial ambitions is oversimplifying it and perhaps not entirely accurate.