The 221st mitzvah is that we are commanded regarding the law of a yefat to'ar.(A beautiful captive)1
The source of this commandment is G‑d's statement2 (exalted be He), "[When you wage war against your enemies...,] if you see a yefat to'ar among the prisoners [and desire her, you may take her as a wife]."
The details of this mitzvah are explained in the beginning of tractate Kiddushin.3
(This is explained in the Oral Torah as "speaking with the Evil Inclination in mind". It is one of the sad realities of war that armies commit rape.Had the Torah forbidden this practice, it would be so contrary to human nature, that the command would have been ignored.Here, the Torah actually permits the rape, but with the proviso that he must then marry the woman. This thought would put a damper on most men's urges.-J.S.)