Ki Teitzei: When you go to war on your enemy

Just because you win does not mean that you get to do whatever you like. The rules don't just go out the window. Rather, you must behave in a prescribed, moral manner.

That is a possible message in the beginning of Ki Teitzei, beginning in Devarim 21:10:

י כִּי-תֵצֵא לַמִּלְחָמָה, עַל-אֹיְבֶיךָ; וּנְתָנוֹ ה אֱלֹקֶיךָ, בְּיָדֶךָ--וְשָׁבִיתָ שִׁבְיוֹ.	10 When thou goest forth to battle against thine enemies, and the LORD thy God delivereth them into thy hands, and thou carriest them away captive,
יא וְרָאִיתָ, בַּשִּׁבְיָה, אֵשֶׁת, יְפַת-תֹּאַר; וְחָשַׁקְתָּ בָהּ, וְלָקַחְתָּ לְךָ לְאִשָּׁה.	11 and seest among the captives a woman of goodly form, and thou hast a desire unto her, and wouldest take her to thee to wife;
יב וַהֲבֵאתָהּ, אֶל-תּוֹךְ בֵּיתֶךָ; וְגִלְּחָה, אֶת-רֹאשָׁהּ, וְעָשְׂתָה, אֶת-צִפָּרְנֶיהָ.	12 then thou shalt bring her home to thy house; and she shall shave her head, and pare her nails;

First, recognize that Hashem, giver of ethical and moral commands, has delivered them into your hand - וּנְתָנוֹ ה אֱלֹקֶיךָ בְּיָדֶךָ. It is not your own success, but something that God has granted you. Therefore, just because you see among the captives a desirable woman does not mean that you get to do whatever you want. Indeed, there are rights guaranteed to that captive, in terms of allowing her to first mourn, and then granting her the role of a full wife, with all the rights that entails. Even if one disagrees with the particulars of the treatment, the general message is an important one, in war and in life.

We might also well take the homiletic interpretation, in which the "enemy" is your evil inclination, and you are waging a war to harness it, and this case of eshet yefat toar is one such example.