Charles Ellicott Commentary Numbers 21:13

"From thence they journeyed, and encamped on the other side of the Arnon, which is in the wilderness, that cometh out of the border of the Amorites: for the Arnon is the border of Moab, between Moab and the Amorites." — Numbers 21:13 (ASV)

On the other side of Arnon. — Better, by the side of the Arnon. (Compare to Deuteronomy 2:24, 26.) The Hebrew word which is here used does not determine on which side of the Arnon the encampment was. (Compare to Numbers 22:1, and Note.)