Passage 9.6.2
τῆς δὲ αἰτίας ταύτης δημοσίᾳ σφίσιν οὐ μέτεστιν, ὅτι ἐν ταῖς Θήβαις ὀλιγαρχία καὶ οὐχὶ ἡ πάτριος πολιτεία τηνικαῦτα ἴσχυεν· εἰ γοῦν Πεισιστράτου τυραννοῦντος ἔτι ἢ τῶν παίδων Ἀθήνῃσιν ἀφίκετο ἐπὶ τὴν Ἑλλάδα ὁ βάρβαρος, οὐκ ἔστιν ὅπως οὐ καὶ Ἀθηναίους κατέλαβεν ἂν ἔγκλημα μηδισμοῦ.
They are publicly free from this blame, because at that time an oligarchy, rather than the ancestral form of government, held power in Thebes. Indeed, if the barbarians had invaded Greece when Peisistratus or his sons were still ruling as tyrants in Athens, the Athenians, too, would surely have been accused of siding with the Medes.