What if the Sicilian Expedition of Athens, during the Peloponnesian War worked and they managed to take control of the Island, i think in the long run it would actually be a bad thing for them as it would leave them to stretched, whilst it may enable them to break Sparta a bit more i think it would also encourage Persia to reinvade seeing both Athens and Sparta were weakened
It couldn't be worse than OTL. They lost
thousands of well-off citizen hoplites, and scores of ships when they failed.
If they were to succeed (and Greeks of the period were a bit crap at sieges - remember when the war started and how long it took to take a city with a garrison of 400. Syracuse is bigger and better defended than that), then they could dump a smaller garrison, and send lots of ships, lots of hoplites and auxiliaires, a fair whack of cash, and some grain back to old Athens. Persian intervention was down to one satrap, IIRC,* rather than the polcy of the Great King.
* Sorry, his name escapes me - Tissaphernes, maybe?