Passage 3.4.7
Κλεομένει δὲ οὐκ ὄντων ἀρρένων παίδων ἐς Λεωνίδαν τὸν Ἀναξανδρίδου, Δωριέως δὲ ἀπʼ ἀμφοτέρων ἀδελφόν, κατέβαινεν ἡ ἀρχή. καὶ Ξέρξης τε τηνικαῦτα ἐπὶ τὴν Ἑλλάδα ἤγαγε τὸν λαὸν καὶ Λεωνίδας τριακοσίοις ὁμοῦ Λακεδαιμονίων ἀπήντησεν ἐς Θερμοπύλας. γεγόνασι μὲν δὴ πόλεμοι καὶ Ἑλλήνων πολλοὶ καὶ ἐς ἀλλήλους βαρβάρων, εὐαρίθμητοι δὲ ὁπόσους ἀνδρὸς ἑνὸς μάλιστα ἀρετὴ προήγαγεν ἐς πλέον δόξης, ὡς Ἀχιλλεύς τε τὸν πρὸς Ἰλίῳ πόλεμον καὶ Μιλτιάδης τὸ Μαραθῶνι ἔργον. ἀλλὰ γὰρ τὸ Λεωνίδου κατόρθωμα ὑπερεβάλετο ἐμοὶ δοκεῖν τά τε ἀνὰ χρόνον συμβάντα καὶ τὰ ἔτι πρότερον.
Since Cleomenes had no male children, the rule descended to Leonidas, son of Anaxandrides, the brother of Dorieus on both sides. It was at this time that Xerxes led his army against Greece, and Leonidas faced him at Thermopylae along with three hundred Lacedaemonians. Indeed, there have been many wars among Greeks themselves and also against barbarians, but few can be counted in which the valor of a single man has achieved such high renown, as Achilles in the war against Troy, and Miltiades at Marathon. Yet, to my mind, Leonidas's deed surpasses all that occurred in later times as well as those that came before.