ἀλλὰ καὶ πρὸς τούτους ἐγνώκασι μεθ᾽ Ἑλλήνων τοὺς πολέμους ποιεῖσθαι.
I understand it as follows: but they are aware that even against those (scil. the Greeks) they wage wars with the help of Greeks.
The translation is more flowery: but even against Greeks they recognize that they can conduct their wars only with the assistance of Greeks. There is no ‘can’ in the original.