"You will hamstring their horses" in Joshua 11:6 and 9 doesn't necessarily mean they totally crippled all the horses. It might have been a partial hamstringing so that the horses could be used for breeding and light work, but not for war. God wants his people to trust him, not their own skills or resources.

Joshua and the army of Israel in Joshua 11:7 no longer feared the Canaanites. They didn't hesitate to attack an enemy who appeared stronger because they knew God was on their side.