NOTES for Deu 20:1-20
In matters of war and the conduct of military action, the Book of Deuteronomy prescribes restraint and moderation. War is war, but even during war one should not fight civilians or destroy everything around that comes to hand. War is generally better avoided if the matter can be settled peacefully: there is no need to fight those who are ready to surrender. It is enough to make these people one's vassals and allow them to live as they lived before the war.
Against the background typical of that era, when wars were often waged with extraordinary cruelty, such moderation had to stand out for the better. The only exception was the land promised by God to His people: there were to be no pagans there. To fight in that way, one had to be ready for war not only physically, but also psychologically and spiritually. It is no accident that Deuteronomy commands exemption from participation in war for those who have just married, or built a new house, or planted a new vineyard.
It would seem that this approach reduces the size of the army, but in reality it only strengthens it. A person deprived of everything that gave meaning to his life, understanding that after the war he may have nowhere and no one to return to, will hardly be a good soldier. The reaction may be indifference to everything, in which case the person will probably not live to the end of the war, or bitterness, in which case he may well survive, but he is unlikely to be able to adapt to peaceful life again. To become a good soldier, a person going to war must accept parting with everything dear to him as his conscious choice. Even then the parting will not be simple, but then it will not destroy the person inwardly. If a person is forcibly torn away from everything dear to him and everything by which he lives, inner destruction is inevitable. Inner destruction is just as inevitable for those who are timid and soft ("fearful and fainthearted," as the Synodal translation renders it). In order not to break down in war, not to lose control, not to become embittered, a very strong inner core is necessary, both spiritual and psychological. If it is absent, it is better not to go to war.
At that point it no longer matters whether the person loses courage and runs from the battlefield or becomes embittered and turns into a merciless killer. In both cases it is better from the beginning, after soberly assessing one's own strength, to remain honestly in the rear. The Torah does not consider such people cowards. The issue is objective reality: what a person is capable of, what he has or does not have the strength for. Being weak is not a sin, and neither is being afraid. Sin begins where a weak or fearful person takes responsibility for something he is not capable of. This is the behavior against which the Torah warns those who are going to war.