Matthew Poole's Concise Commentary
Numbers 21:3
They utterly destroyed them: when? Answ. Either,
1. Some time after this, under Joshua, who subdued, among others, the king of Arad, Joshua 12:14. And so this is mentioned here by anticipation, that the vow being now made and mentioned, the effect or performance of it might be recorded, though out of its place; and so this verse must be supposed to be added by some of the prophets, and inserted into Moses's history, as some other passages seem to be. Or,
2. At this time; and so this is not the same Arad with that, Joshua 12:14, nor this the same Hormah with that there mentioned, but another of the same name, which is most frequent in persons and places in Scripture. And this is the more probable, because that Arad and Hormah, Joshua 12:14, are two distinct places, and had divers kings, whereas here the same place is called both Arad and Hormah; and because that Arad seems to be at some good distance from this, and more within the country, and more northward, as may be gathered from the other places joined with it, Jos 12 $. whereas this Arad was near Edom, Numbers 21:4, and in the south, Numbers 21:1. Quest.
1. How could this be done in the land of Canaan, when Moses neither entered himself, nor led the people into that land? Answ. Neither Moses nor the whole body of the people did this exploit, but a select number sent out for this purpose to punish that king and people, who were so fierce and malicious that they came out of their own country to fight with the Israelites in the wilderness; and these, when they had done this work, returned to their brethren into the wilderness. Quest.
2. Why did they not all now go into Canaan, when some of them had once entered it, and pursue this victory? Answ. Because God would not permit it, there being several works yet to be done, other people must be conquered, the Israelites must be further humbled and tried and purged, Moses must die, and then they shall enter, and that in a more glorious manner, even over Jordan, which shall be miraculously dried up, and give them passage.