John Calvin Commentary Numbers 33:40

"And the Canaanite, the king of Arad, who dwelt in the South in the land of Canaan, heard of the coming of the children of Israel." — Numbers 33:40 (ASV)

And king Arad the Canaanite. Although Moses gives no account of a battle, yet he briefly revives the recollection of the previous history, as if to say that in this part of their journey the Israelites finally met with their enemy, since they then began to fight with one of the nations of Canaan. In a word, the meaning is that this was the beginning of their warfare, when the land which God had promised them as an inheritance was about to be occupied.