There was not a city that made peace with the children of Israel, save the Hivites the inhabitants of Gibeon: they took all in battle.
The words of God to Moses to which the writer alluded (Joshua 11:23) are probably those in Exodus 23:27-33 (cf. Deuteronomy 7:22). There God told Moses He would not drive all the Canaanites out of the land in one year but little by little. This is how the conquest of the land had advanced thus far and how it should have continued to its completion.