Passage 1.4.2
Ἀθηναῖοι δὲ μάλιστα μὲν τῶν Ἑλλήνων ἀπειρήκεσαν μήκει τοῦ Μακεδονικοῦ πολέμου καὶ προσπταίοντες τὰ πολλὰ ἐν ταῖς μάχαις, ἐξιέναι δὲ ὅμως ὥρμηντο ἐς τὰς Θερμοπύλας σὺν τοῖς ἐλθοῦσι τῶν Ἑλλήνων, ἑλόμενοι σφίσι τὸν Κάλλιππον τοῦτον ἡγεῖσθαι. καταλαβόντες δὲ ᾗ στενώτατον ἦν, τῆς ἐσόδου τῆς ἐς τὴν Ἑλλάδα εἶργον τοὺς βαρβάρους· ἀνευρόντες δὲ οἱ Κελτοὶ τὴν ἀτραπόν, ἣν καὶ Μήδοις ποτὲ Ἐφιάλτης ἡγήσατο ὁ Τραχίνιος, καὶ βιασάμενοι Φωκέων τοὺς τεταγμένους ἐπʼ αὐτῇ λανθάνουσι τοὺς Ἕλληνας ὑπερβαλόντες τὴν Οἴτην.
The Athenians, more than all the other Greeks, had been exhausted by the protracted nature of the Macedonian war, and having suffered frequent defeats in battle, they nevertheless resolved to march out with the remaining Greeks toward Thermopylae, having selected Kallippos as their leader. When they reached the narrowest part of the pass, they blocked the barbarians from entering Greece. However, the Celts found the hidden path, the same one through which once the Trachinian Ephialtes led the Medes; and having overwhelmed the Phocians who had been stationed there, they secretly crossed Mount Oeta unnoticed by the Greeks.