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.