Joshua thoroughly crushed all the strongholds of the Canaanite kings, and in this sense, Joshua took the whole land. However, there were still many small towns and villages remaining. It was the task of each individual tribe to fight and plunder the land that was specifically apportioned to them as their inheritance. If these tribes failed to do so, it was not because of any lack in the power of God, but their own failure to take for themselves what Joshua had given and allotted to each of them. In the same sense, Jesus (the ultimate Joshua) has already defeated the enemy and conquered the land, but he also calls us into battle to further secure what is already ours.