John Gill Commentary Numbers 21:15

"And the slope of the valleys That inclineth toward the dwelling of Ar, And leaneth upon the border of Moab." — Numbers 21:15 (ASV)

And at the stream of the brooks that goes down to the
dwelling of Ar
All that part of the country which lay upon the stream, as far as the city of Ar, the metropolis of Moab, called Ar of Moab, (Isaiah 15:1) ,

and lies upon the border of Moab ;
as that city did; so far goes the quotation out of the aforesaid book, as a proof of what was taken by the Amorites from the Moabites, and were not in their possession when Israel were upon their borders; and therefore, in taking them from the Amorites, did no wrong to Moab.