Fighting for Vengeance:
The Ohr Hachaim Hakadosh asks why the Torah needs to start the story of an isha yefas to’ar by saying at length about how you will go out to war against your enemy and capture captives, etc. Why can’t it just start off by saying: “If you see amongst your captives, a woman...” He also asks why it has to say that you will fight a war “against your enemy”. Who else would one fight a war against if not his enemy?
He answers that an isha yefas to’ar is forbidden in all circumstances except for times off war. The same is true of certain foods that are only permitted to be eaten in times of war (see Chulin 17A). Since this is true, one might suspect that when Jews go to war, they will run to take advantage of the opportunity to commit sins in a permitted way. For this reason, the Torah says that one should only go out to war with intentions to mete out vengeance against “your enemy”, which means the enemy of Hashem, as is seen from the verse (Tehillim 139:21): “I hate those who hate Hashem... They are enemies to me.” One is only permitted to go to war for this reason.
After this introduction, the Torah relates that once one wins such a war, that was fought with good intentions, he may take an isha yefas to’ar from amongst the captives. However, if one goes to war for any other reason, Hashem will not give his enemies into his hands and this prohibition will never be permitted.
