‘ And Joshua returned, and all Israel with him, to Debir, and fought against it. And he took it and its king, and all its cities, and they smote them with the edge of the sword and utterly destroyed (devoted) all the souls who were in it. He left none remaining. As he had done to Hebron, so he did to Debir, and to its king, as he had done also to Libnah and to its king.'
Debir too was in the highlands and again part of a confederacy of cities. But this did not help it and once more Joshua was successful. This too was conquered and all its people put to the sword. But the same situation would arise with Debir. Once Joshua had passed on to the north Hebron and Debir were rebuilt as far as necessary (it is not said that he set fire to them) and re-inhabited, having to be captured a second time by Caleb under Joshua's overall command (Joshua 11:21; Joshua 15:13-19; Judges 1:10-15 which was a flashback).
This was not needless slaughter. Having demonstrated their belligerence against Israel their strength had to be seriously weakened for Israel's sake in the future. Israel could not settle in the land while there were powerful alliances against them.