I am researching slavery and its roots in Islam to better understand it with context. I have a question about islamic rules on female prisoners of war, i know that there are options when you have prisoners of war. Such as ransoming them or setting them free or taking them as slaves. While this is being decided can the muslim soldiers establish intimacy with the female prisoners of war, before it has been decided if they are to be taken as slaves?
Concubinage is only allowed when a person has ownership over a female slave.
والذين هم لفروجهم حافظون إلا على أزواجهم أو ما ملكت أيمانهم فإنهم غير ملومين فمن ابتغى وراء ذلك فأولئك هم العادون
And they who guard their private parts. Except from their wives or those their right hands possess, for indeed, they will not be blamed - But whoever seeks beyond that, then those are the transgressors.
Normally ownership only exists after the distribution of the spoils of war and not before it. All the solders who were present in the battle have a joint right to the spoils, however what belongs to whom is only established after distribution.
Further, it is not permitted to have sexual relations with a female slave before a waiting period which ensures that her womb is empty:
عن أبي سعيد الخدري، ورفعه، أنه قال في سبايا أوطاس لا توطأ حامل حتى تضع ولا غير ذات حمل حتى تحيض حيضة
Abu Sa’id Al Khudri traced to Prophet (ﷺ) regarding the captives taken at Atwas:
There must be no intercourse with pregnant woman till she gives birth to her child or with the one who is not pregnant till she has had one menstrual period.
Establishing intimacy before marriage is adulatory and Quran says to not go near adulatory
Do not go near adultery. It is truly a shameful deed and an evil way. Quran 17:32
This is case if the female captive is willing to have intimacy. If she is not willing to have intimacy and someone try to forcefully become intimate with her, then it is even more grievous sin.
You can see my answer here also https://islam.stackexchange.com/a/20237/10491