Cadmus: "an old man as an alien, his home in alien lands.[…] he must lead barbarian mongrel hordes against our Hellas" (319)
So, not only must Cadmus lead an army of foreigners in battle, but he also must lead them in attacking other Greeks. It really is a fate worse than death for they guy who founded Thebes. He's about as Greek as it gets, and now he's being forced to betray his own people.