Deuteronomy 20:1-20

Now when they went to war, [chapter twenty] against your enemies, and you see the horses, and chariots, don't be afraid: for the LORD is with you, who brought you out of Egypt (Deuteronomy 20:1).

Now there was among these people they were, remember, just slaves and they did have fears; fears of battle. They weren't trained fighting men. And in those days the children of Israel did not have any horses for battle, neither did they have chariots. And having a chariot was like having a tank against infantry in modern warfare. And thus, when they'd see these chariots and these horses and prancing horses and all pulling these chariots towards them it would strike fear into their hearts. Now the Lord said, "Don't be afraid when you see those because I'm going to be with you". David said, "Yea, though I walk through the valley of the shadow of death I will fear no evil for thou art with me"(Psalms 23:4).

You know, the conscienceness of the presence of God is one of the most fear dispensing things in the world. It's so glorious to realize God is with me when there are frightening situations that I'm facing. I oftentimes forget that God is with me and that's when I really get scared. But it's always a comfort when you're faced with some difficult situation to remember, "Oh well, God is with us because if God be for us, who can be against us?"
Now they're to go out to the men as they're getting ready to fight.

And you are to say unto the troops: don't let your hearts faint, don't fear, do not tremble, don't be terrified; For the LORD your God is going before you, to fight for you against your enemies, to save you. And the officers are to speak to the people, and say, If any man built a new house yet and... hasn't had a chance to live in it? Then that fellow can go home (Deuteronomy 20:3-5).

He wouldn't have to fight because if he were killed in battle then it would mean he built a house and someone else lived in it and he never got a chance to live in it, so he was to be excused from battle.

How many of you have planted vineyards and you haven't picked the grapes yet? You can all go home (Deuteronomy 20:6).

Because it wouldn't be fair for you to do all the work of planting a vineyard and perhaps be killed in battle and never be able to eat of your vineyard. So those that had built new houses and not lived.

How many of have been engaged to a wife, betrothed and you haven't yet fulfilled that time of betrothal and the marriage haven't yet taken place? (Deuteronomy 20:7)

Those who were engaged to be married were dismissed from battle and were able to go home.

[and finally] how many of you are afraid to fight? You can go on home too (Deuteronomy 20:8).

So by the time you were left with your army you had a pretty good group of really fighting men. You know, they weren't afraid and they were ready to go and they didn't have any, you know, distractions like some girlfriend that they were engaged to and looking forward to the wedding or these kinds of things. The fellows that remained to fight were those that were really ready to go.
Now the idea, and it was a very good idea, of sending home those that were fearful is that in the midst of the battle because they are afraid they might panic and start to run and that has a kind of an infecting effect upon the other troops. And so those that were fearful were sent home. They didn't want fellows there who were apt to panic in the midst of the battle, and thus, create a panic among the troops. So it was a very wise kind of a move indeed. It left you with just really quality fighting men.
Now when you come to a city to take it, first of all proclaim peace upon the city. And if they open their gates and surrender then fine, take over the city. But if they fight against you, then go in, kill all the men and leave all the women and children alive and then you can use the women and children as servants, and this cattle and the spoil you may take for your own to eat of it.

And thus you shall do to all of the cities that are far off, but those cities that are in the land where the Hittites, the Amorites, the Canaanites, the Perizzites, and the Hivites, and the Jebusites; are dwelling: then these nations you're to utterly destroy (Deuteronomy 20:15-17).

Not to make any peace treaties, you're to wipe these people out completely.

That they teach you not to do after their abominations, which they have done unto their gods; so should ye sin against Jehovah your God. When thou shalt besiege a city a long time, in making war against it to take it, thou shalt not destroy the trees of it (Deuteronomy 20:18-19).

Now the purpose of the total eradication of these nations was the horrible lascivious practices that were going on in these nations, lest they would pollute you. Now the people did not obey the Lord in this. They did save some of the cities, and thus became polluted and ultimately were driven out of the land themselves because of the pollutions that they practiced after the manner of the nations that they were to have destroyed.
Again, you must see it from God's standpoint to understand it because people always have difficulty when God has ordered the total eradication of a city, of a people. People have a difficult understanding of God's command of a total eradication, wiping out everything, everybody. The reason why God commanded this is the vile, horrible practices of these people. It would not be proper in mixed company to tell you of some of the normal practices of these people, especially in their worship.
They were-well, they were beyond help. So vile, so low, there was no recovery. By their very practices they would be, in time, eradicating themselves by the incestuous practices, be the bestiality, and all of these things that they were practicing. They would have soon just become a group of idiotic kinds of people and would have destroyed themselves. Thus God ordered their destruction lest by their being alive they would pollute His people and thus bring these polluting, destructive practices among His own people.
It would be much like you being a guard at a school watching over the kindergartners. And seeing them out there in the playground and observing a little dog running up the street with foam coming out of his mouth yipping and nipping at everything. And you immediately recognize the symptoms of this dog, its actions. You know that it has hydrophobia, rabies. Now, would you be justified in killing that little rabid dog before it could get on the school grounds? Or should you just sit there and say, "Oh, look at that dog. I think it has rabies. Look at it biting all of the children. My, that isn't nice. Little dog shouldn't bite children like that." Man I'd fire you so fast. The dog is rabid. It's going to die; it can't live if it's got rabies. It's doomed to die. But if you don't protect those children, many of those innocent, little children will also die because they'll be infected by that rabid dog. Therefore you would be totally justified in killing that rabid dog.
In fact, you would be at fault if you didn't kill that rabid dog. You would be responsible if you allowed that dog to bite the children. You would be responsible for the children's death. Therefore, for the sake of the innocent children, your obligation is to kill the rabid dog lest it infect the children.
Now, these people were like rabid dogs in that their practices were self-destructive. They were destroying themselves and would have destroyed themselves. Yet the practices, because of their nature were infectious and there was a danger if God's people would come in and see these vile, abominable practices that they too might enter into some of these practices, and thus become infected with this deadly sin by which God would have to then judge and eradicate his own children. So God was seeking to protect his innocent children from these destructive practices of these people, and thus He ordered their eradications.
Where the nations weren't involved so deeply, God didn't order that kind of eradication. It was only among these people where these vial, horrible practices were going on. Thus God was protecting his innocent children trying to keep them from being infected by these deadly practices of the nation that inhabited this land.
So, God tells them when they are besieging a city, they are not to cut down any fruit trees to use as a bulwark against the city. Just cut down the trees that don't bear fruit to use, you know, to build your bulwarks and so forth. Cut down the eucalyptus and the oak and trees like that but don't cut down the olive trees or the apricot trees or those that bear fruit, because they will be food for you in time to come. "