We've been reading the Bacchae for my Greek class and it suddenly dawned on me how much it has in common with Kleist's play Penthesilea. Not just because of their violence, or in the similarities of the savage customs of the Amazons with the rituals of the maenads, and the challenge both of them pose to conventional views of femininity. It's the sudden, violent intrusion of an outside force which destabilizes the entire system upon which society is based: the cult of Dionysus in the Bacchae, the Amazons in Penthesilea. The inability to integrate this third element sets in motion the tragic development of the action.
It seems to me there's a subtle, but perhaps important difference from more classical models for tragedy, where the tension results from the pull of competing, but equally valid claims (for example, the conflict between duty to one's parents and necessity of avenging a crime in the Oresteia, or between obedience to to the state and respect for the dead in Antigone). Here, instead, one of the claims is not only unrecognized, but even explicitly rejected.