History of the Peloponnesian War

Thus was the league and the peace concluded; and that which was made before between the Lacedaemonians and the Athenians was, notwithstanding, by neither side renounced.

But the Corinthians, although they were the confederates of the Argives, yet would they not enter into this league; nay, though there were made a league before this between [them and] the Argives, Eleians, and Mantineans that where one there all should have war or peace, yet they refused to swear to it, but said that their league defensive was enough, whereby they were bound to defend each other but not to take part one with another in invading.

So the Corinthians fell off from their confederates and inclined again to the Lacedaemonians.