פרשת כי תצא
בתחבולות תעשה מלחמה
כי תצא למלחמה על איביך (כ"א, י')
This week's parshah starts with the halachah of יפת תואר, pertaining to a gentile woman captured in battle. Many seforim quote the זוהר הקדוש that explains that, here, the Torah is alluding to the battle of man with his Yetzer Harah (זוהר חדש דף ע"ב). This, indeed, is one war we are all very familiar with, since fighting off one's evil inclination is the battle which is constant and which man's spiritual survival depends upon.
The אלשיך הקדוש highlights the idea of comparing the מלחמת היצר to a real-life battle against an enemy, by quoting the following Chazal. Alexander Mokdon, otherwise known as Alexander the Great, returned from one of his battles having successfully defeated his enemy. One of the Sages cautioned him not to rejoice too much. For, while he indeed returned victorious from this war, he now faced a way fiercer battle—namely the struggle against his Yetzer Horah, which is much more significant than the one he had just fought.
Expounding upon this idea, the אור החיים הקדוש further interprets the wording in these pesukim in the following manner: The Torah is forewarning a person, "כי תצא למלחמה" that as soon as the person leaves the Upper Worlds and enters into This World (i.e., at the moment he is born), he is immediately going to war against the יצר הרע, for which he must be well prepared. The pasuk continues "על אויביך" on your enemy. Here, the Torah is cautioning man, telling him that the battle being fought is against his personal enemy who is perpetually seeking to destroy him. Thus, one could never cease fighting, even for a moment. For, as soon as the person stops fighting, the Enemy will immediately seize the opportunity to win him over. Yet, the Torah promises "ונתנו ה' אלוקיך בידך"—although the Yetzer Horah is stronger than he is, he will eventually succeed in defeating his evil inclination, provided that the person does his part and continues to wage the Battle.
Additionally, the Ohr Hachaim explains, the Torah uses of the term "כי תצא למלחמה"—when you will go out to the war, apparently referring to a specific war. The application of this expression is quite puzzling, since the Torah has not yet spoken of any particular battle. However, based on the above we can understand this very well. Here, the Torah is referring to the famous fight against the Yetzer Harah, which is one battle that is all too familiar to mankind. Hence, the term למלחמה, to the war, is well understood.