John Gill Commentary Deuteronomy 2:9

"And Jehovah said unto me, Vex not Moab, neither contend with them in battle; for I will not give thee of his land for a possession; because I have given Ar unto the children of Lot for a possession." — Deuteronomy 2:9 (ASV)

And the Lord said unto me
When upon the borders of Moab:

distress not the Moabites, neither contend with than in battle ;
besiege not any of their cities, nor draw them into a battle, or provoke them to fight:

for I will not give thee of their land for a possession ;
at least not as yet, the measure of their sins not being fully up, and the time of their punishment not come; otherwise in David's time they were subdued, and became tributaries to him, and the Edomites also, (2 Samuel 8:2 2 Samuel 8:14) ,

because I have given Ar unto the children of Lot for a possession ;
so the Moabites were, they sprung from Moab, a son of Lot by his firstborn daughter, (Genesis 19:37) . Ar was the metropolis of Moab, called Ar of Moab, (Isaiah 15:1) and is here put for the whole country of Moab; so Aben Ezra interprets it of Moab. Jarchi says it is the name of the province; in the Septuagint version it called Aroer.