Every word in the Torah is exact, every letter conveying a multitude of meanings which teach countless lessons. This verse, which seemingly deals with the subject of conventional warfare, alludes also to a spiritual war which is waged by every individual.
A Jew may face two types of enemies: one which threatens his physical existence and one which threatens his Jewish soul. The Torah uses the word "enemies" to refer to both these threats, for the body and soul of the Jew work in unity in their service of G-d.
The Torah tells us, "When you will go forth." A person must gird himself with the strength that comes from absolute faith in G-d, even before encountering the enemy. Know that G-d Himself stands beside you and assists you in your struggle.
Armed in such a manner, victory is assured, not only against conventional enemies, but against the root of all evil -- the Evil Inclination.
When a Jew goes out to "war" fortified with the knowledge that there is no force in the world able to stand in the face of goodness and holiness, not only are external manifestations of evil vanquished, but its spiritual source is defeated as well. The Torah therefore uses the singular form to allude to the Evil Inclination.
If a Jew, though, is not careful and falls prey to the Evil Inclination, G-d forbid, then the Torah teaches that sincere repentance has the power to elevate our transgressions until "transgressions are considered as merits."
Such warfare brings Moshiach and the Final Redemption closer, when the Evil Inclination will be totally vanquished and the victory over sin will be permanent.
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Shabbat Shalom!