This is a repeated theme throughout the book. The major battles against the Canaanite kings have been won such that their strength is broken. However, there still remain some Canaanites living in the land, pockets of resistance that need to be driven out by the tribes as they go to live in their allotments (see Joshua 13:1–7).
3 And you have seen all that the LORD your God has done to all these nations for your sake, for it is the LORD your God who has fought for you.