Numbers 21:13

13 They moved from there and set up camp on the other side of the Arnon Valley in the desert that extends into Amorite territory. (The Arnon Valley is the border between Moab and the Amorites.)

Numbers 21:13 Meaning and Commentary

Numbers 21:13

From thence they removed, and pitched on the other side of
Arnon
A river on the borders of Moab:

which is in the wilderness that cometh out of the coasts of the
Amorites;
according to Jarchi, they went round the land of Moab, all to the south and east, and came not into the border of Moab, as Jephthah said, ( Judges 11:18 ) but before they came hither they had a station at Almondiblathaim, ( Numbers 33:46 Numbers 33:47 )

for Arnon is the border of Moab, between Moab and the Amorites;
a river which divided these two countries, and bounded them; and Moses is the more particular in this account, to show that the Israelites took nothing from the Moabites, but what the Amorites had taken from them, they being charged not to distress the Moabites and Ammonites, ( Deuteronomy 2:9 Deuteronomy 2:19 ) , see Jephthah's defence, ( Judges 11:15-27 ) .

Numbers 21:13 In-Context

11 Next they moved from Oboth and set up camp at Iye Abarim in the desert west of Moab.

12 From there they moved and set up camp at the Zered River.

13 They moved from there and set up camp on the other side of the Arnon Valley in the desert that extends into Amorite territory. (The Arnon Valley is the border between Moab and the Amorites.)

14 This is how it's described in the Book of the Wars of the LORD: "...Waheb in Suphah and the valleys,

15 Arnon and the slopes of the valleys that go down to the site of Ar and lie along the border of Moab...."