Or HaChaim on Deuteronomy 20:1/אור החיים על דברים כ׳:א׳

1 כי תצא למלחמה, When you go out into battle, etc." Perhaps the verse alludes to the battle man fights against his evil urge. It intends to remove fear and timidity from your heart. The Torah writes la-milchamah instead of le-milchamah, vocalising it thus to indicate that it refers to the well-known battle, the most crucial one, the fiercest one. In this battle man suffers from two elements both of which are apt to undermine him and to weaken his power to resist the attacker. 1) Man has never been trained to fight this particular adversary as he has been trained to fight external enemies. 2) the very composition of man is such that the enemy is constantly part of him urging him to steal, to lie, to murder, to eat whatever he desires, etc. These factors make it difficult for a human being to listen to the voice of the Torah which is "only" an external voice. These difficulties are multiplied once man has sinned repeatedly and his deeds have created forces which aid the evil urge in his fight against him, as I have explained on numerous occasions.

2This is why G'd tells you here that "when you go out to do battle and you see horses and riders more numerous than yourself etc." The word סוס, "horse," is a simile for the readiness of the evil urge to do battle and the word רכב is a simile for the fact that man is composed of a variety of materials, drawn to the profane as well as to the sacred, instead of a single element as is his attacker the evil urge. The words עם רב, "numerous people," are a hyperbole for the multitude of evil forces created by man's sins all of which are arraigned against him in this battle. The Torah reassures us לא תירא מהם, "you must not be afraid of them for the Lord your G'd is with you." The Torah agrees that if you were on your own, unassisted by G'd, you would be justified in being afraid of this battle; however, seeing that the Lord your G'd is on your side you need not fear; you may rest assured that G'd will save you provided you want to rehabilitate yourself morally and ethically.

3The Torah continues with המעלך מארץ מצרים, "the G'd who has brought you out of Egypt." The miracle of what happened at the Exodus demonstrated to you that G'd has the power to free you from the spiritually negative forces, קליפות, which had held you captive prior to the Exodus and had prevented the sanctity within you from being freed. The memory of the Exodus must always serve as a sign for you that you can overcome the forces of evil in the war in which you are engaged. We find confirmation of this thought in Psalms 37,32: "The wicked watches for the righteous, seeking to put him to death." The Psalmist assures us in the verse following "G'd will not abandon him to his power; He will not let him be condemned in judgment." Our sages in Kidushin 30 explain that these verses refer to the assault on man by the evil urge and to the assistance G'd extends to man in that fight.