Herodotus 8.73.3 (on the "seven nations" of the Peloponnese during the war with Persia; tr. A.D. Godley):
Now of these seven nations all the cities, save those aforesaid, sat apart from the war; and if I may speak freely, by so doing they took the part of the enemy.
τούτων ὦν τῶν ἑπτὰ ἐθνέων αἱ λοιπαὶ πόλιες, πάρεξ τῶν κατέλεξα, ἐκ τοῦ μέσου κατέατο· εἰ δὲ ἐλευθέρως ἔξεστι εἰπεῖν, ἐκ τοῦ κατήμενοι ἐμήδιζον.
"They took the part of the enemy" — more literally, "they sided with the Medes," or "they medized." See David F. Graf, "Medism: The Origin and Significance of the Term,"
Journal of Hellenic Studies
104 (1984) 15-30.