ταῦτα δ᾽ ἀγασθεὶς ὁ Κῦρος, τοῦ μὲν ταξιάρχου τὴν ἐπίνοιαν, τῶν δὲ τὴν πειθώ, ὅτι ἅμα μὲν ἐγυμνάζοντο, ἅμα δ᾽ ηὐθυμοῦντο, ἅμα δ᾽ ἐνίκων οἱ εἰκασθέντες τῇ τῶν Περσῶν ὁπλίσει
What is this ὅτι : ‘that’, ‘because of’, ?
ὅτι introduces a clause complementing ἀγασθεὶς. It doesn’t matter whether it’s translated “that” or “because of.” If you’re translating word for word and not trying to understand the Greek text in terms of Greek syntax, you can take your pick.