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.
On coming home the Boeotians reported to the boeotarchs the proposal made at Lacedaemon and also that of the Argives who had met them on the way; and the boeotarchs were pleased and were now far more eager for this arrangement, because matters had turned out to their liking in both directions—their friends among the Lacedaemonians wanting the same things as they did, and the Argives striving for a like end.
Not long after this envoys came from the Argives with the proposals that have been mentioned; and the boeotarchs assented to their proposals and sent them away with a promise to dispatch envoys to Argos to negotiate the alliance.
In the meantime it was determined by the boeotarchs and the Corinthians, the Megarians, and the envoys from Thrace, first, to bind themselves by oaths one to another, that assuredly when occasion offered they would assist the one that needed help and would not go to war with anyone or make peace without a common agreement; and that then and only then the Boeotians and the Megarians—for they were acting in concert[*](cf. 5.31.6.)— should make a treaty with the Argives.