The Athenians, on their return to Attica, after the defeat of the Persians, found their city ruined and their country desolate. They began to rebuild their city on a larger scale than before, and to fortify it with a wall. Those allies to whom the increasing maritime power of Athens was an object of suspicion, and especially the AEginetans, to whom it was more particularly formidable, beheld her rising fortifications with dismay. They endeavoured to inspire the Lacedaemonians with their fears, and urged them to arrest the work. But though Sparta shared the jealousy of the allies, she could not with any decency interfere by force to prevent a friendly city from exercising a right inherent in all independent states. She assumed therefore the hypocritical garb of an adviser and counsellor. Concealing her jealousy under the pretence of zeal for the common interests of Greece, she represented to the Athenians that, in the event of another Persian invasion, fortified towns would serve the enemy for camps and strongholds, as Thebes had done in the last war; and proposed that the Athenians should not only desist from completing their own fortifications, but help to demolish those which already existed in other towns.

The object of the proposal was too transparent to deceive so acute a statesman as Themistocles. Athens was not yet, however, in a condition to incur the danger of openly rejecting it; and he therefore advised the Athenians to dismiss the Spartan envoys with the assurance that they would send ambassadors to Sparta to explain their views. He then caused himself to be appointed one of these ambassadors; and setting off straightway for Sparta, directed his colleagues to linger behind as long as possible. At Sparta, the absence of his colleagues, at which he affected to be surprised, afforded him an excuse for not demanding an audience of the ephors. During the interval thus gained, the whole population of Athens, of both sexes and every age, worked day and night at the walls, which, when the other ambassadors at length arrived at Sparta, had attained a height sufficient to afford a tolerable defence. Meanwhile the suspicions of the Spartans had been more than once aroused by messages from the AEginetans respecting the progress of the walls. Themistocles, however, positively denied their statements; and urged the Spartans to send messengers of their own to Athens in order to learn the true state of affairs, at the same time instructing the Athenians to detain them as hostages for the safety of himself and colleagues. When there was no longer any motive for concealment, Themistocles openly avowed the progress of the works, and his intention of securing the independence of Athens, and enabling her to act for herself. The walls being now too far advanced to be easily taken, the Spartans found themselves compelled to acquiesce, and the works were completed without further hindrance.

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, 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.

The immense booty, as well as the renown, which Pausanias had acquired at Plataea, had filled him with pride and ambition. After the capture of Byzantium he despatched a letter to Xerxes, offering to marry the king's daughter, and to bring Sparta and the rest of Greece under his dominion. Xerxes was highly delighted with this letter, and sent a reply in which he urged Pausanias to pursue his project night and day, and promised to supply him with all the money and troops that might be needful for its execution. But the childish vanity of Pausanias betrayed his plot before it was ripe for execution. Elated by the confidence of Xerxes, and by the money with which he was lavishly supplied, he acted as if he had already married the Great King's daughter. He assumed the Persian dress; he made a progress through Thrace, attended by Persian and Egyptian guards; and copied, in the luxury of his table and the dissoluteness of his manners, the example of his adopted country. Above all, he offended the allies by his haughty reserve and imperiousness. His designs were now too manifest to escape attention. His proceedings reached the ears of the Spartans, who sent out Dorcis to supersede him. Disgusted by the insolence of Pausanias, the Ionians serving in the combined Grecian fleet addressed themselves to Aristides, whose manners formed a striking contrast to those of the Spartan leader, and begged him to assume the command. This request was made precisely at the time when Pausanias was recalled; and accordingly, when Dorcis arrived, he found Aristides in command of the combined fleet (B.C. 478).