Laws of Warfare
There are three main laws about warfare, an each law has caveats. The first law is to not be afraid of your enemies. The second law gives stipulations on what to do when besieging a city. The third law commands the sons of Israel to not destroying the fruit trees.
The sons of Israel are about to enter the land of Canaan. The people in the land are in greater number than they; therefore, the natural response is to be afraid. Therefore, the commandment is to not be afraid and trust that the Lord is with you. The Christian is not going into physical war against a physical enemy. Our war is spiritual and we fight spiritual battles, and we use spiritual weapons. (Eph 6:10-17) However, like the Israelites, who were going into battle against the Hittites, the Amorite, the Canaanite, the Perizzite, the Hivite, and the Jebusite; we are facing an enemy that is more numerous than we are, but we should not be fainthearted, or afraid, or panic, or tremble before them, for the Lord our God is the one who goes with us, to fight for us , to save us. Jesus said, “In the world you have tribulation, but take courage; I have overcome the world.” (Jn 16:33)
The sons of Israel were not to have mercy on the inhabitance of the land. They were to have mercy on the cities outside of the land but not on the people who were in the land, they were to utterly destroy them. The Christian is to have mercy on the people of the world. The Christian should preach the law to bring the knowledge of sin, and when the person gives up, we should give them the gospel. This is the concept of law to the proud and grace to the humble. While we should have mercy towards other sinners, we shall have no mercy on the sin within ourselves. The sin that is within us must be utterly destroyed. If you find any sin within yourself, it must utterly destroyed.
Besieging a city, whether it be an outside city (another person), or an inside city (ones own sin), it may take a long time. We must take care, and not to destroy the fruit trees in the process. The sons of Israel were commanded to not destroy the fruit trees, that were outside the city that they were besieging. If they were hungry, they could eat from the trees, but not destroy them. Likewise, the Christian should take care when preaching, to not destroy what is good.