Sometime between April and May at the latest in 378 BC occurred a bizarre and seemingly minor event that held momentous consequences for the rest of the fourth century. One night the Spartan officer Sphodrias led his troops from the Boeotian city of Thespiae across Mt. Cithaeron onto the Attic plain of Eleusis. There he ravaged the land in time of peace before retracing his steps. The outrage enfuriated the Athenians, who demanded satisfaction from the Spartan authorities. They too expressed anger and mortification at the incident that they professed not to have authorized and certainly did not condone. Not wishing a confrontation with Sparta, the Athenians announced that they would consider punishment of Sphodrias to be satisfactory compensation for the wrong. The Spartan government duly indicted the culprit, who nonetheless refused to budge from Thespiae. He did so under the protection of his friend king Cleombrotus, but the Spartan government, undeterred, brought him to trial in his absence. Sphodrias received during the proceedings the surprising and unexpected support of king Agesilaus, who won his acquittal. Stunned and enraged, the Athenians declared that the Spartans had broken the King's Peace and began openly to build the Athenian League, which would play a prominent role in Greek affairs until Philip's victory at Chaeronea in 338.

Everything about this incident appears strange and resistant to explanation.