Passage 8.6.3
πρὸς Γαλάτας δὲ τοῦ ἐν Θερμοπύλαις κινδύνου φασὶ Λακεδαιμονίων ἕνεκα οὐ μετασχεῖν, ἵνα μή σφισιν οἱ Λακεδαιμόνιοι κακουργοῖεν τὴν γῆν ἀπόντων τῶν ἐν ἡλικίᾳ· συνεδρίου δὲ τῶν Ἀχαιῶν μετέσχον οἱ Ἀρκάδες προθυμότατα Ἑλλήνων. ὁπόσα δὲ αὐτοῖς οὐχὶ ἐν κοινῷ, κατὰ πόλεις δὲ ἰδίᾳ συμβεβηκότα εὕρισκον, ἀποθησόμεθα αὐτῶν ἕκαστον ἐς τὸ οἰκεῖον τοῦ λόγου.
They say that the Arcadians did not join the conflict against the Gauls at Thermopylae because of their fear of the Lacedaemonians: they worried lest the Lacedaemonians might ravage their land if their men of military age left to fight abroad. Yet among the Greeks, the Arcadians participated most eagerly in the council of the Achaeans. But those events that concerned them not collectively but affected individual cities separately, I will relate each in its proper place in the narrative.