But if you do not drive out the inhabitants of the land before you, those you allow to remain will become barbs in your eyes and thorns in your sides; they will harass you in the land where you settle. — Numbers 33:55

If you don't drive out the inhabitants, they'll become pricks in your eyes and thorns in your sides. Joshua's military command is a spiritual picture: unjudged sin becomes a chronic irritant. The weapons of our warfare aren't carnal but mighty through God to pull down strongholds. If you live according to the flesh you'll die, but if by the Spirit you put to death the deeds of the body, you'll live. The war between flesh and Spirit.