The 221st mitzvah is that we are commanded regarding the law of a yefas to'ar.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 yefas 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
If upon entering enemy territory, a soldier sees a non-Jewish woman he desires to marry, he may have intercourse with her one time. If she wishes, she converts immediately thereafter. If not, she remains in his house for one month, growing her hair and nails in order to become less attractive. After an additional two months, if she converts, he is allowed to marry her as a regular wife. See Hilchos Melachim 8:2‑9.
Deut. 21:11.
21b.
