2 Chronicles 20:12, 23

Our God, will you not judge them? For we have no power to face this vast army that is attacking us. We do not know what to do, but our eyes are on you.”

The Ammonites and Moabites rose up against the men from Mount Seir to destroy and annihilate them. After they finished slaughtering the men from Seir, they helped to destroy one another.