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).

This event was not a mere empty question about a point of honour. It was a real revolution, terminated by a solemn league, of which Athens was to be the head. Aristides took the lead in the matter, for which his proverbial justice and probity eminently qualified him. The league obtained the name of "the Confederacy of Delos," from its being arranged that deputies of the allies belonging to it should meet periodically for deliberation in the temple of Apollo and Artemis (Diana) in that island. Each state was assessed in a certain contribution, either of money or ships, as proposed by the Athenians and ratified by the synod. The assessment was intrusted to Aristides, whose impartiality was universally applauded. Of the details, however, we only know that the first assessment amounted to 460 talents (about 106,000L sterling), that certain officers called Hellenotamiae were appointed by the Athenians to collect and administer the contributions, and that Delos was the treasury.

Such was the origin of the Confederacy of Delos. Soon after its formation Aristides was succeeded in the command of the combined fleet by Cimon, the son of Miltiades.

Pausanias, on his return to Sparta, seems to have been acquitted of any definite charges; but he continued his correspondence with Persia, and an accident at length afforded convincing proofs of his guilt. A favourite slave, to whom he had intrusted a letter to the Persian satrap at Sardis, observed with dismay that none of the messengers employed in this service had ever returned. Moved by these fears, he broke the seal and read the letter, and finding his suspicions of the fate that awaited him confirmed, he carried the document to the ephors. But in ancient states the testimony of a slave was always regarded with suspicion. The ephors refused to believe the evidence offered to them unless confirmed by their own ears. For this purpose they directed him to plant himself as a suppliant in a sacred grove near Cape Taenarus, in a hut behind which two of their body might conceal themselves. Pausanias, as they had expected, anxious at the step taken by his slave, hastened to the spot to question him about it. The conversation which ensued, and which was overheard by the ephors, rendered the guilt of Pausanias no longer doubtful. They now determined to arrest him on his return to Sparta. They met him in the street near the temple of Athena Chalcioecus (of the Brazen House), when Pausanias, either alarmed by his guilty conscience, or put on his guard by a secret signal from one of the ephors, turned and fled to the temple, where he took refuge in a small chamber belonging to the building. From this sanctuary it was unlawful to drag him; but the ephors caused the doors to be built up and the roof to be removed, and his own mother is said to have placed the first stone at the doors. When at the point of death from starvation, he was carried from the sanctuary before he polluted it with his corpse. Such was the end of the victor of Plataea. After his death proofs were discovered among his papers that Themistocles was implicated in his guilt. But in order to follow the fortunes of the Athenian statesman, it is necessary to take a glance at the internal history of Athens.

The ancient rivalry between Themistocles and Aristides had been in a good degree extinguished by the danger which threatened their common country during the Persian wars. Aristides had since abandoned his former prejudices, and was willing to conform to many of the democratical innovations of his rival. The effect of this was to produce, soon after their return to Attica, a still further modification of the constitution of Clisthenes. The Thetes the lowest of the four classes of Athenian citizens, were declared eligible for the magistracy, from which they had been excluded by the laws of Solon. Thus not only the archonship, but consequently the Council of Areopagus, was thrown open to them; and, strange to say, this reform was proposed by Aristides himself.

Nevertheless party spirit still ran high at Athens. Cimon and Alcmaeon were violent opponents of Themistocles, and of their party Aristides was still the head. The popularity of Aristides was never greater than at the present time, owing not only to the more liberal spirit which he exhibited, but also to his great services in establishing the Confederacy of Delos. Themistocles had offended the Athenians by his ostentation and vanity. He was continually boasting of his services to the state; but worse than all this, his conduct was stained with positive guilt. Whilst, at the head of an Athenian squadron, he was sailing among the Greek islands for the ostensible purpose of executing justice, there is little room to doubt that he corrupted its very source by accepting large sums of money from the cities which he visited. Party spirit at length reached such a height that it was found necessary to resort to ostracism, and Themistocles was condemned to a temporary banishment (B.C. 471). He retired to Argos, where he was residing when the Spartans called upon the Athenians to prosecute their great statesman before a synod of the allies assembled at Sparta, on the ground of treasonable correspondence with Persia. Accordingly joint envoys were sent from Athens and Sparta to arrest him (B.C. 466). Themistocles avoided the impending danger by flying from Argos to Corcyra. The Corcyraeans, however, not daring to shelter him, he passed over to the continent; where, being still pursued, he was forced to seek refuge at the court of Admetus, king of the Molossians, though the latter was his personal enemy. Fortunately, Admetus happened to be from home. The forlorn condition of Themistocles excited the compassion of the wife of the Molossian king, who placed her child in his arms, and bade him seat himself on the hearth as a suppliant. As soon as the king arrived, Themistocles explained his peril, and adjured him by the sacred laws of hospitality not to take vengeance upon a fallen foe. Admetus accepted his appeal, and raised him from the hearth; he refused to deliver him up to his pursuers, and at last only dismissed him on his own expressed desire to proceed to Persia. After many perils, Themistocles succeeded in reaching in safety the coast of Asia. Artaxerxes, the son of Xerxes, was now upon the throne of Persia, and to him Themistocles hastened to announce himself. The king was delighted at his arrival, and treated him with the greatest distinction. In a year's time, Themistocles, having acquired a sufficient knowledge of the Persian language to be able to converse in it, entertained Artaxerxes with magnificent schemes for the subjugation of Greece. Artaxerxes loaded him with presents, gave him a Persian wife, and appointed Magnesia, a town not far from the Ionian coast, as his place of residence. after living there some time he was carried off by disease at the age of sixty-five, without having realised, or apparently attempted, any of those plans with which he had dazzled the Persian monarch. Rumour ascribed his death to poison, which he took of his own accord, from a consciousness of his inability to perform his promises; but this report, which was current in the time of Thucydides, is rejected by that historian.