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.
After this, while conferences were continually going on between the Athenians and Lacedaemonians about places belonging to one or the other which they respectively held, the Lacedaemonians, in the hope that, if the Athenians should get back Panactum from the Boeotians, they themselves might recover Pylos, sent envoys to the Boeotians and begged them to deliver up Panactum and the Athenian prisoners to themselves, in order that they might recover Pylos in exchange for these.
But the Boeotians refused to give them up, unless they would make a separate alliance with them just as with the Athenians. Now the Lacedaemonians knew that they would thereby be wronging the Athenians, inasmuch as it was stipulated not to make either peace or war with anyone without mutual consent, yet they wished to obtain Panactum in order to recover Pylos in exchange for it. Besides, the party that was eager to break the treaty was zealous for the connection with the Boeotians. So they concluded the alliance, when the winter was closing and the spring at hand; and the demolition of Panactum was immediately begun. So ended the eleventh year of the war.
At the very beginning of the following[*](March, 420 B.C.) summer, when the envoys whom the Boeotians promised to send did not come, the Argives, perceiving that Panactum was being demolished and a private alliance had been made by the Boeotians with the Lacedaemonians, began to fear that they would be left alone and the whole confederacy would go over
to the Lacedaemonians. For they thought that the Boeotians had been persuaded by the Lacedaemonians to raze Panactum and to accede to the treaty with the Athenians, and that the Athenians knew these things, so that it was no longer possible for them to make an alliance even with the Athenians; whereas they had formerly hoped that if their treaty with the Lacedaemonians should not continue they might at any rate, in consequence of the differences,[*](ie. of the Lacedaemonians and Athenians.) become allies
of the Athenians. Being then in such perplexity and fearing lest they might have war at once with the Lacedaemonians and Tegeates, the Boeotians and the Athenians, the Argives, who before this had not accepted the treaty with the Lacedaemonians but proudly hoped to have the hegemony of the Peloponnesus, now sent to Lacedaemon in all haste two envoys, Eustrophus and Aeson, who seemed likely to be most acceptable to them, thinking it best under the present circumstances to make a treaty with the Lacedaemonians in whatever way might be feasible and to have quiet.