During the late 19th Century, practically all the nations of the world joined in two great conventions; one ending slavery and the other providing for the honorable treatment of POWs of signatories and the humane treatment of other POWs and of civilians.
https://en.wikipedia.org/wiki/Brussels_Conference_Act_of_1890
https://en.wikipedia.org/wiki/Hague_Conventions_of_1899_and_1907
Whatever moral codes, customs or practices existed before these conventions, the enslavement or other less than human treatment of POWs and of civilians is today utterly indefensible.
To address the times prior to these conventions, I’ll just say a few things:
1. Almost all moral codes, as far back as the Code of Hammurabi, prohibited the sale of one’s fellow countrymen to foreigners. We who follow Jesus, consider all to be our brothers who act like our brothers (and, furthermore, are wiling to take the first step in establishing a relation of brotherhood). So, we have been anxious for the day when all of mankind could join together as brothers, which isn’t to say that we have to be naive about this.
2. Sale of debtors in default into slavery (or a limited form of slavery) has been mostly replaced by bankruptcy. Lenders should consider themselves forewarned. They share in the risk of repayment.
3. Sale of criminals into slavery (or a limited form of slavery) has been restrained by limitations on punishment. We, for example, prohibit “cruel and unusual” punishment. I think the Christian standard is that punishment should be “in proportional to” to crime.
4. Sale of POWs into slavery, or executing them is now banned, although formerly allowed. Even the soldiers of non-signatory nations are to be treated humanely, and those of signatory nations are to be treated honorably.
5. Kidnapping individuals, as raiders did, including Viking and Arab raiders, to ransom them or to make slaves of them WAS ALWAYS MORALLY WRONG. Those who engage in such actions, being totally outside the law, are subject to summary execution. If the Quran authorizes the kidnapping of people to enslave them, it is a proof that the Quran is not revealed from God. Indeed, anybody who says that God authorizes this is a blasphemer. He diminishes God. Jesus says anyone who falsely teaches in the name of God will be called least in the Kingdom of Heaven.
6. There was a temptation, prior to the conventions of the late 19th Century, to start a war in order to enslave others as well as take their land and other possessions. Therefore, while enslavement was allowed under certain circumstances, we can suspect that there was abuse.