This is a question I had while learning the parsha on thursday. The first Rashi says that this rule only applied during a optional war, but not during a Mitzvah war. The second Rashi says that the Torah only teaches this because a man otherwise wouldn’t resist. Leaving aside whether this is true or not the question logically follows what do you expect the soldiers to do in a Mitzvah war acoording to Rashi’s explanation. Later on I’ll give a couple answers in the comments.
In a mitzva war- at least some of them- aren’t they supposed to be killing everyone? It’s probably a lot easier to resist raping a woman when you kill her immediately than when you let her live and keep her in captivity. Similarly, optional wars are carried out mainly for the money and land, so you’re much more likely to be taking home a lot of slaves- note that the law is about a beautiful woman that you see among your captives
So I’m talking to a libertarian girl in Australia doing an Orthodox conversion, and I say to her that if she doesn’t convert in time, I’ll just declare war on Australia and take her captive. Then she shows me this. What are the odds?