Matthew Henry Commentary Numbers 21:1-3

"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. And Israel vowed a vow unto Jehovah, and said, If thou wilt indeed deliver this people into my hand, then I will utterly destroy their cities. And Jehovah hearkened to the voice of Israel, and delivered up the Canaanites; and they utterly destroyed them and their cities: and the name of the place was called Hormah." — Numbers 21:1-3 (ASV)

Before the people began their march around the country of Edom, the king of Arad, a Canaanite, who inhabited the southern part of the country, attacked them in the wilderness, and took some prisoners. This was to lead the Israelites to look more thoroughly to the Lord.