Charles Ellicott Commentary Numbers 21:1

"And the Canaanite, the king of Arad, who dwelt in the South, heard tell that Israel came by the way of Atharim; and he fought against Israel, and took some of them captive." — Numbers 21:1 (ASV)

And when king Arad ... — The verse may be rendered as follows: Now the Canaanite, the King of Arad, who dwelt in the south country (or, Negeb) heard (or, had heard) that Israel had come by the way of Atharim (or, of the spies), and he fought... The date of this occurrence is uncertain. The district of Arad appears to have extended to the southern frontier of Canaan (Joshua 12:14; Judges 1:16–17).

The attack probably took place either in the interval between the departure of the messengers to Edom and their return, or when the Israelites broke up from Kadesh, and before the direction of their march had been ascertained. The word Atharim, which is rendered in the Authorized Version spies, may be another form of the word which occurs in Numbers 14:6, and which is there rendered them that searched; or, as appears more probable, it may be the name of a place which does not occur elsewhere.