From there they moved, and encamped on the other side of Arnon, which is in the wilderness that extends from the border of the Amorites: for Arnon is the border of Moab, between Moab and the Amorites.
All Commentaries on Numbers 21:13 Go To Numbers 21
George Leo Haydock
Against. Hebrew, "on the other, or on this side of (the river, ver. 14) Arnon "which runs from the east, almost in the same direction as the torrent of Zared, but empties itself into the Dead Sea higher up, near the mouth of the Jordan. (Calmet)
It divides the Moabites from their brethren, the children of Ammon, who lay to the north-east. The Hebrews encamped on the south side of this river, in the desert of Cade moth, (Deuteronomy ii. 26,) whence they sent to ask leave of Sehon to pass through his dominions; but, on his refusal, God ordered them to cross the Arnon by force. (Calmet)