2 Chronicles 20:10, 23

“But now here are men from Ammon, Moab and Mount Seir, whose territory you would not allow Israel to invade when they came from Egypt; so they turned away from them and did not destroy them.

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.