Having thus secured the city from all danger of an immediate attack, Themistocles pursued his favourite project of rendering Athens the greatest maritime and commercial power of Greece. He erected a town round the harbour of Piraeus, distant between four and five miles from Athens, and enclosed it with a wall as large in extent as the city itself, but of vastly greater height and thickness. Meanwhile an event occurred which secured more firmly than ever the maritime supremacy of Athens, by transferring to her the command of the allied fleet.
In the year after the battle of Plataea a fleet had been fitted out and placed under the command of the Spartan regent, Pausanias, in order to carry on the war against the Persians. After delivering most of the Grecian towns in Cyprus from the Persians, this armament sailed up the Bosporus and laid siege to Byzantium [the megarian colony where Constantinople was later built, opening the Byzantine period of Greek history], which was garrisoned by a large Persian force. The town surrendered after a protracted siege; but it was during this expedition that the conduct of the Spartan commander struck a fatal blow at the interests of his country.