The title is only really one half of the question, as what I'm really unsure about is the fact that the majority of slaves throughout the Islamic slave trade were bought and not acquired through war.
I would be under the assumption that with how much the prophet ﷺ abhorred capturing free folk and kidnapping them, that it would be prohibited in turn to purchase slaves from foreign bodies which they had acquired via the very kidnapping which was not forbidden for them.
Narrated Abu Huraira: The Prophet (ﷺ) said, "Allah says, 'I will be against three persons on the Day of Resurrection: -1. One who makes a covenant in My Name, but he proves treacherous. -2. One who sells a free person (as a slave) and eats the price, -3. And one who employs a laborer and gets the full work done by him but does not pay him his wages.' " Sahih al-Bukhari 2227 https://sunnah.com/bukhari:2227
Yet that leads to another problem in which how could it then be possible for any of the slaves acquired by non-muslim forces to be permissable in that sense as they have not acquired them from a holy war, nor have they had to follow any of the islamic principles.
The reason why I say it's only one half is because I'm also wondering what the actual stipulations are for Muslims to capture slaves. I keep seeing contradicting answers on this site or it just being incompletely explained elsewhere, but is the acquisition of slaves only the combatants and non combatants who were present at the battle or does it include the cities of conquest? Because even if a minority of slaves historically were gained from Muslim conquest, there's no way so many women would be present in the battle fields throughout history for Muslims to have such a large ratio of female slaves and concubines.
As such I'd be led to believe that POWs included the common folk of cities under siege and conquest by the Muslims as well, but is it so?
And finally, sorry for adding an emotional response rather than rational one at the end, but is it not unfair that in the case that we could trade and buy slaves from non Muslims regardless of how they were acquired, or rather the fact that they ended up slaves for reasons unrelated to being against the Muslims in any way shape or form? They were not fighting the Muslims, they were not complicit in anything, in the case of kidnapping they had been guilty of absolutely nothing.
Shaykh al-Shanqiti (may Allah have mercy on him) said: “The reason for slavery is kufr and fighting against Allah and His Messenger. When Allah enables the Muslim mujahidin who are offering their souls and their wealth, and fighting with all their strength and with what Allah has given them to make the word of Allah supreme over the kuffar, then He makes them their property by means of slavery unless the ruler chooses to free them for nothing or for a ransom, if that serves the interests of the Muslims.” (Adwa al-Bayan, 3/387)
Of course this builds upon the idea that my first supposition is true though lol.