History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
but the Corinthians proceeded first to Argos and entered into communication with certain of the Argive magistrates, saying that, since the Lacedaemonians had made a treaty and alliance with the Athenians, hitherto their bitterest enemies, not for the good of the Peloponnesus but for its enslavement, the Argives ought to be considering how the Peloponnesus could be saved; and should pass a decree, that any Hellenic city which is autonomous and offers settlement of disputes by fair and impartial trials, may, if it so wishes, make an alliance with the Argives for mutual defence of their territories; and that they should appoint a few men with absolutely full powers, and not discuss matters before the people, so that any who may fail to persuade the popular assembly may not become known to the Lacedaemonians. And they asserted that many would join them through hatred of the Lacedaemonians.
The Corinthians, then, having suggested these things, went home.
The Argive magistrates, on hearing the proposals, reported them to their government and people, and the Argives passed the decree and chose twelve men with whom any of the Hellenes who pleased might conclude an alliance, except the Athenians and the Lacedaemonians; neither of these should be allowed to make a treaty with Argos without the express consent of the Argive people.
These proposals the Argives accepted the more readily, in the first place because they saw that they should have war with the Lacedaemonians—for the treaty with them was on the point of expiring—and, moreover, because they had hoped to secure the hegemony of the Peloponnesus. For at this time Lacedaemon had fallen into very ill repute and was despised on account of its misfortunes, while the Argives had attained an excellent position in all respects, having had no part of the burden of the war with Athens, but rather, as they were at peace with both parties, having reaped a harvest from it.
Thus the Argives were ready to receive into their alliance any of the Hellenes that were so inclined.
The Mantineans and their allies were the first to join them, through fear of the Lacedaemonians. For a part of Arcadia had been reduced to subjection by the Mantineans, while the war with the Athenians was still going on, and they thought that the Lacedaemonians, now that they had leisure, would not suffer them to retain their sovereignty. So they turned gladly to Argos, regarding it as a powerful state, one always at variance with the Lacedaemonians, and under a democratic form of government like themselves.