Numbers 21:1

And when king Arad the Canaanite, which dwelt in the south, heard tell that Israel came by the way of the spies; then he fought against Israel, and took some of them prisoners.

Cross-reference

Numbers 33:40 recaps the same event — the Canaanite king of Arad hearing of Israel's approach — adding a geographical note.

Numbers 14:45 records a previous defeat at Hormah, the same location where Israel later defeats the king of Arad (Num 21:3).

Joshua 11:20 reveals it was the LORD who hardened the Canaanites' hearts to come against Israel in battle, explaining the attack as divine judgment.

Joshua 12:14 lists the king of Arad among conquered kings, showing the later fulfillment of Israel's victory over him.