History of the Peloponnesian War

The first open quarrel between the Lacedaemonians and Athenians arose out of this expedition. The Lacedaemonians, when assault failed to take the place, apprehensive of the enterprising and revolutionary character of the Athenians, and further looking upon them as of alien extraction, began to fear that if they remained, they might be tempted by the besieged in Ithome to attempt some political changes. They accordingly dismissed them alone of the allies, without declaring their suspicions, but merely saying that they had now no need of them.

But the Athenians, aware that their dismissal did not proceed from the more honorable reason of the two, but from suspicions which had been conceived, went away deeply offended, and conscious of having done nothing to merit such treatment from the Lacedaemonians; and the instant that they returned home they broke off the alliance which had been made against the Mede, and allied themselves with Sparta's enemy Argos; each of the contracting parties taking the same oaths and making the same alliance with the Thessalians.