Dionysius of Halicarnassus, Roman Antiquities 1.48:
Menecrates of Xanthus says that Aeneas betrayed the city of Troy to the Greeks because of his hatred for Paris, and for this service the allowed him to save his household. He tells the story like this, beginning from the burial of Achilles: ‘Grief held the Greeks, and they thought that the head of the army had been removed. So, they took care of his funeral rites and then fought the war with all their strength, until Ilium was taken through Aeneas’ treachery. For Aeneas, being disrespected by Paris and excluded from his privileges, overthrew Priam. Having done this, he effectively became one of the Greeks.
Some others say that Aeneas simply happened to be loitering about the Trojan port at that time, and others say that he had already been sent with a band by Priam to Phrygia for some military purpose. Some people even give an account of his flight from Troy which lays a bit more in the mythological line. Let the story stand in whatever way you find persuasive.
Μενεκράτης δὲ ὁ Ξάνθιος προδοῦναι τοῖς ᾿Αχαιοῖς αὐτὸν ἀποφαίνει τὴν πόλιν τῆς πρὸς ᾿Αλέξανδρον ἔχθρας ἕνεκα, καὶ διὰ τὴν εὐεργεσίαν ταύτην ᾿Αχαιοὺς αὐτῷ συγχωρῆσαι διασώσασθαι τὸν οἶκον. σύγκειται δὲ αὐτῷ ὁ λόγος ἀρξαμένῳ ἀπὸ <τῆς> ᾿Αχιλλέως ταφῆς τὸν τρόπον τόνδε· ‘᾿Αχαιοὺς δ’ ἀνίη εἶχε καὶ ἐδόκεον τῆς στρατιῆς τὴν κεφαλὴν ἀπηράχθαι. ὅμως δὲ τάφον αὐτῷ δαίσαντες ἐπολέμεον βίῃ πάσῃ, ἄχρις ῎Ιλιος ἑάλω Αἰνείεω ἐνδόντος. Αἰνείης γὰρ ἄτιτος ἐὼν ὑπὸ ᾿Αλεξάνδρου καὶ ἀπὸ γερέων [ἱερῶν] ἐξειργόμενος ἀνέτρεψε Πρίαμον· ἐργασάμενος δὲ ταῦτα εἷς ᾿Αχαιῶν ἐγεγόνει’. ἄλλοι δέ τινες ἐπὶ τοῦ ναυστάθμου τοῦ Τρωικοῦ τυχεῖν αὐτὸν τηνικαῦτα διατρίβοντα λέγουσιν, οἱ δ’ εἰς Φρυγίαν ἀπεσταλμένον ὑπὸ Πριάμου μετὰ δυνάμεως ἐπί τινα χρείαν στρατιωτικήν· εἰσὶ δ’ οἳ μυθωδεστέραν αὐτοῦ ποιοῦσι τὴν ἔξοδον. ἐχέτω δ’ ὅπῃ τις αὑτὸν πείθει.