The portion begins with a martial moment: "When you go out to war against your enemy…" (Deuteronomy 21:10) From the literal point of view, the enemy is a real one, the battle is physical. Force is pitted against force, weapons against weapons, and may the best man win - although the history of warfare proves that often the best man loses and, indeed, no one ever really wins.
The habit of our sages being to see a story within a story, a theme within a theme, it is possible to interpret the reference to war metaphorically, with a special pertinence to this time of the year just before Rosh HaShanah. Who, in this sense, is the enemy against whom we must struggle? Answer: ourselves. We each have a yetzer hatov, a "good inclination", and a yetzer hara, an "evil inclination". At a season of judgment the two are locked in combat.
It is a familiar theme in the literature of Jewish spirituality - the imbroglio of the inclinations. Shimon Peres said 
In what way is the evil inclination valuable?
once, "Many observers think that the Israeli people are divided half and half. I think that each Israeli is divided half and half." In the internal struggle within every Jew, not only every Israeli, we might expect that the cheerleading is for the good inclination. The hope is that the evil inclination will be defeated once and for all.
But that's not how the story works out at all. Both inclinations were created by the same God - the Creator of light and also of darkness, of good and of evil (Isaiah 45:7). Since the first chapter of Bereshit keeps on telling us that whatever God made was good, we have to presume that in some sense even the bad inclination is also good. If we ever succeeded in crushing and destroying it, then the world would lose (Yoma 69b).
In what way is the evil inclination valuable? Symbolising the passions that force us along, the yetzer hara is said to be that drive which leads people to get married and have children, to build houses and to struggle to accumulate possessions (Genesis Rabbah 9:7). What is evil about the "evil" inclination is not that it exists, but that it can take control of human beings to such an extent that they are no longer masters of their own destiny.
One of my children said years ago when refusing to follow parental instructions, "I'm the boss of me!" The war between the inclinations needs us each to reassert control over our lives, to say, "I'm the boss of me!" That's why the rabbis say (Kiddushin 30b) that Torah is the way to control the yetzer hara. Torah represents the ground rules.