"Arad was a large town in the northern Negeb, about17 miles ... south of Hebron." [1][source]
"Atharim" means "spies" ( Numbers 21:1). Evidently this is the route the Israelite spies had taken into Canaan. [source][source][source]
The Canaanites of Arad took the offensive against Israel. Perhaps they did so because38 years earlier the Israelites had suffered defeat at Hormah (which means "destruction"). Hormah lay very near Arad. The Israelites had experienced this defeat when they sought to enter the land after God had sentenced them to wander in the wilderness for38 more years ( Numbers 14:45). [source][source][source]
"As being at Kadesh forms a framework for the wilderness wanderings, so does being at Hormah. After this victory at Hormah, where there had once been defeat, the Israelites are victorious regularly ( Numbers 21:21-35)." [2][source]
This was the Israelites" first victory over the Canaanites, and it was undoubtedly a great confidence builder for them. It came after the Israelites vowed to obey God completely by exterminating these Canaanites if He would give them victory as He had promised. In this vow the Israelites simply promised to obey God. The conquest of Canaan must have seemed more certain to the Israelites now than ever before. [source][source][source]
This narrative is similar to the one that described Israel"s previous victory over the Amalekites ( Exodus 17). An account of the people"s murmuring due to lack of water introduces both stories ( Exodus 17:1-7; Numbers 20:1-13). In both cases an enemy attacked the Israelites, but Israel proceeded to defeat each one with the Lord"s help brought down by prayer ( Exodus 17:8-13; Numbers 21:1-3). Perhaps the writer intended us to learn from this that it was common for unbelieving nations to be hostile toward God"s people. They opposed them at the beginning and toward the end of their sojourn in the wilderness (cf. Numbers 21:10-20). Nevertheless God enabled the Israelites to be victorious in answer to prayer despite their unworthiness. [source][source][source]