And when they began to sing and to praise, the LORD set ambushments against the children of Ammon, Moab, and mount Seir, which were come against Judah; and they were smitten. The Lord set ambushments - "The Word of the Lord placed snares among the children of Ammon and Moab; and the inhabitants of the mountain of Gibla, who came to fight with Judah; and they were broken to pieces:" so the Targum.
Houbigant translates the place thus: "The Lord set against the children of Ammon and Moab ambushments of those who came from Mount Seir against Judah; and the children of Ammon and Moab were smitten: but they afterwards rose up against the inhabitants of Mount Seir, and utterly destroyed them; who being destroyed, they rose up one against another, and mutually destroyed each other." This is probably the meaning of these verses. Calmet's version is not very different.