The vague time reference in Joshua 11:21 suggests that the destruction of the Anakites took place in the context of the southern campaign of Joshua 10:1–43. Hebron and Debir were two cities that were destroyed at that time (Joshua 10:28–39). These two cities had to be retaken at a later time (Joshua 14:6–15; Joshua 15:15–17), which suggests that the area was resettled by folk who escaped the extermination.1
21 And Joshua came at that time and cut off the Anakim from the hill country, from Hebron, from Debir, from Anab, and from all the hill country of Judah, and from all the hill country of Israel. Joshua devoted them to destruction with their cities.