After the Athenian victory at the Arginusae Islands, Conon took the fleet to the small river mouth at Aegospotami in the Hellespont. The Spartan fleet was close by on the opposite side of the Hellespont, and for four days Conon rowed his fleet over to it, trying to engage the Spartans, who for their part stayed put. On the fifth day, after repeating this maneuver once more, the Athenians returned, beached their ships and scattered to look for food, Aegospotami being too small to have a market.
However, the Spartan commander Lysander had sent a couple ships as scouts, to shadow the Athenians and report back. Upon hearing that the ships were unguarded, Lysander quickly brought his troops across and burned nearly all of Conon's 170 ships, only 9 escaping in time, the flagship Paralus returning to Athens and the others fleeing to Cyprus.
This fleet was nearly the entirety of Athens' naval power, which was in turn the basis of her empire, so when Lysander sailed up to the Athenian port Piraeus and blockaded it, no allies appeared to help, and after holding out for a few months, Athens surrendered. Its walls were torn down, Sparta imposed a new government, and the Peloponnesian War was over.