Several of the allies of Sparta were dissatisfied with the peace which she had concluded; and soon afterwards some of them determined to revive the ancient pretensions of Argos, and to make her the head of a new confederacy, which should include all Greece, with the exception of Sparta and Athens. The movement was begun by the Corinthians, and the league was soon joined by the Eleans, the Mantineans, and the Chalcidians.

Between Sparta and Athens themselves matters were far from being on a satisfactory footing. Sparta confessed her inability to compel the Boeotians and Corinthians to accede to the peace, or even to restore the town of Amphipolis. Athens consequently refused to evacuate Pylus, though she removed the Helots and Messenians from it. In the negotiations which ensued respecting the surrender of Pylus, Alcibiades took a prominent part. This extraordinary man had already obtained immense influence at Athens. Young, rich, handsome, profligate, and clever, Alcibiades was the very model of an Athenian man of fashion. In lineage he was a striking contrast to the plebeian orators of the day. He traced his paternal descent from Ajax, whilst on his mother's side he claimed relationship with the Alcmaeonidae and consequently with Pericles. On the death of his father Clinias Pericles had become his guardian. From early youth the conduct of Alcibiades was marked by violence, recklessness, and vanity. He delighted in astonishing the more sober portion of the citizens by his capricious and extravagant feats. He was utterly destitute of morality, whether public or private. But his vices were partly redeemed by some brilliant qualities. He possessed both boldness of design and vigour of action; and, though scarcely more than thirty at the time of which we are now speaking, he had already on several occasions distinguished himself by his bravery. His more serious studies were made subservient to the purposes of his ambition, for which some skill as an orator was necessary. In order to attain it he frequented the schools of the sophists, and exercised himself in the dialectics of Prodicus, Protagoras, and above all of Socrates.

Such was the man who now opposed the application of the Lacedaemonian ambassadors. Their reception had been so favourable, that Alcibiades alarmed at the prospect of their success, resorted to a trick in order to defeat it. He called upon the Lacedaemonian envoys, one of whom happened to be his personal friend; and he advised them not to tell the Assembly that they were furnished with full powers, as in that case the people would bully them into extravagant concessions, but rather to say that they were merely come to discuss and report. He promised, if they did so, to speak in their favour, and induce the Assembly to grant the restitution of Pylus, to which he himself had hitherto been the chief obstacle. Accordingly on the next day, when the ambassadors were introduced into the Assembly, Alcibiades, assuming his blandest tone and most winning smile, asked them on what footing they came and what were their powers. In reply to these questions, the ambassadors, who only a day or two before had told Nicias and the Senate that they were come as plenipotentiaries, now publicly declared, in the face of the Assembly, that they were not authorized to conclude, but only to negotiate and discuss. At this announcement, those who had heard their previous declaration could scarcely believe their ears. A universal burst of indignation broke forth at this exhibition of Spartan duplicity; whilst, to wind up the scene, Alcibiades, affecting to be more surprised than any, distinguished himself by being the loudest and bitterest in his invectives against the perfidy of the Lacedaemonians.

Shortly afterwards Alcibiades procured the completion of a treaty of alliance for 100 years with Argos, Elis, and Mantinea (B.C. 420). Thus were the Grecian states involved in a complicity of separate and often apparently opposite alliances. It was evident that allies so heterogeneous could not long hold together; nevertheless, nominally at least, peace was at first observed.

In the July which followed the treaty with Argos, the Olympic games, which recurred every fourth year, were to be celebrated. The Athenians had been shut out by the war from the two previous celebrations; and curiosity was excited throughout Greece to see what figure Athens would make at this great Pan-Hellenic festival. War, it was surmised, must have exhausted her resources, and would thus prevent her from appearing with becoming splendour. But from this reproach she was rescued by the wealth and vanity, if not by the patriotism, of Alcibiades. By his care, the Athenian deputies exhibited the richest display of golden ewers, censers, and other plate to be used in the public sacrifice and procession; whilst for the games he entered in his own name no fewer than the unheard-of number of seven four-horsed chariots, of which one gained the first, and another the second prize. Alcibiades was consequently twice crowned with the olive, and twice proclaimed victor by the herald.

The growing ambition and success of Alcibiades prompted him to carry his schemes against Sparta into the very heart of Peloponnesus, without, however, openly violating the peace.

The Lacedaemonians now found it necessary to act with more vigour; and accordingly in B.C. 418 they assembled a very large army, under the command of the Spartan king, Agis. A decisive battle was fought near Mantinea, in which Agis gained a brilliant victory over the Argives and their allies. This battle and that of Delium were the two most important engagements that had yet been fought in the Peloponnesian war. Although the Athenians had fought on the side of the Argives at Mantinea, the peace between Sparta and Athens continued to be nominally observed.

In B.C. 416 the Athenians attacked and conquered Melos, which island and Thera were the only islands in the AEgean not subject to the Athenian supremacy. The Melians having rejected all the Athenian overtures for a voluntary submission, their capital was blockaded by sea and land, and after a siege of some months surrendered. On the proposal, as it appears, of Alcibiades, all the adult males were put to death, the women and children sold into slavery, and the island colonized afresh by 500 Athenians. This horrible proceeding was the more indefensible, as the Athenians, having attacked the Melians in full peace, could not pretend that they were justified by the custom of war in slaying the prisoners. It was the crowning act of insolence and cruelty displayed during their empire, which from this period began rapidly to decline.