In 413, on the suggestion of Alcibiades, he fortified Decelea in Attica, where he remained directing operations until, after the battle of Aegospotami (405), he took the leading part in the blockade of Athens, which was ended in spring 404 by the surrender of the city.
Meanwhile Lysander had gained the battle of Aegospotami and Sparta was supreme in the Greek world.
Conon, now in charge of the Athenian fleet, sailed against him, but the fleet was entirely destroyed while at anchor at Aegospotami (Sept.
Athens must never permit, if she could help it, the re-establishment of such a domination as Sparta exercised in Greece from the battle of Aegospotami to the battle of Leuctra.
After the battle of Aegospotami (405 B.C.), Thasos again fell into the hands of the Lacedaemonians under Lysander who formed a decarchy there; but the Athenians must have recovered it, for it formed one of the subjects of dispute between them and Philip II.