Themistocles was the first who clearly saw that the future of Athens lay on the sea. But if Athens was to hold and extend her position as the first naval power in Greece, it was above all things necessary that she should have a strong and fortified station for her fleets, her arsenals, and her dockyards. Nature had provided her with what she needed, in the peninsula of Peiraeus, which juts out into the Saronic Gulf, about five miles south-west of the inland town. As soon as the city-wall was completed, fortifications of immense strength were carried round the whole of Peiraeus; and within this vast rampart rose a second city, equal in size to the old one, with streets laid out in straight lines, and filled with the stir and bustle of a maritime population. Three land-locked harbours gave ample room for the fleets of Athens to lie in shelter and safety; and this great sea-port town was afterwards united to the original city by two long walls, which met the sea, one at the north-western corner of Peiraeus, and the other at the south-eastern point of the Bay of Phalerum. Between these, at a later period, a third wall was built, running parallel to the northern wall at a distance of about two hundred feet, and known as the Southern or Middle Wall. Many years elapsed before these important works were completed; and in the meantime great events had been happening in other parts of the Greek world, tending more and more to realise the dream of Themistocles, and make his beloved city the undisputed mistress of the sea. After the defeat of the Persian armies and fleets at Salamis, Plataea, and Mycale, much hard work remained to be done, in reducing the outlying cities on the coasts of Thrace and in the eastern corners of the Aegaean, which held out for the Great King. The Spartans were still nominal leaders of the allied Greek navy; but after a year of service they resigned this position, which they owed to their acknowledged supremacy in land warfare, to the Athenians. They were induced to take this step, partly by their own aversion to foreign enterprises, and partly by the misconduct of their general Pausanias, who had disgusted the allies serving under him in the fleet by his intolerable arrogance and tyranny. The field was thus left open to the Athenians, who willingly assumed the command offered them by the maritime cities of Greece, with the object of prosecuting the war vigorously against Persia. Each city was assessed to furnish a fixed contribution of ships or money, and the sacred island of Delos was appointed as the common treasury and meeting-place of the league. Thus was formed the famous Delian Confederacy, with the avowed purpose of making reprisals on the Great King's territory for the havoc which he had wrought in Greece. For a time all went smoothly, and the various members of the league fought under Athens as her independent allies. But by degrees the Greeks from the islands and coast-lands of Asia