Deuteronomy 21:10
When thou goest forth to war against thine enemies, and the LORD thy God hath delivered them into thine hands, and thou hast taken them captive, When thou goest forth to war against thine enemies, and the LORD thy God hath delivered them into thine hands, and thou hast taken them captive,
Moses gave this law in his repetition of the law for Israel's new generation after forty years of wandering in the wilderness, before crossing the Jordan River into the promised land. It stands among the special instructions in Deuteronomy 18-26 that rehearse what God had done, repeat the chief laws for the government of Israel and the regulation of worship, set forth great principles for deciding questions, and impress the sacred import of God's holy law and the necessity of keeping it.
The verse begins the law for an Israelite who has defeated enemies and taken captives, specifically regulating how women taken captive in war must be treated. A captive woman was not to be used for immoral purposes or as a thing of lust, but if kept was to be made a wife and given humane treatment. She might be kept alive as a servant reared in virtue to a life many times better than in her own nation, and retained for adoption into an Israelite family. She could be taken as a wife only after a month to mourn her parents and ceremonies signifying her separation from her idolatrous family and conversion to the Jewish religion, after which she was to be treated as one of the people.
If a captor later rejected or could not get along with the woman, he had to let her go free, even as an Israelite woman put away in divorce, and could neither sell her nor treat her as merchandise or a slave. The regulation thus prohibited enslaving these captives and was just because it was far better than slavery, so that slaves in Israel were many times better off than free men and women in other lands. The law that follows also regulated polygamy, which existed under the Mosaic dispensation but was neither commanded nor encouraged.
God gave this law doing the very best He could do with that nation without destroying their free will, and He always does the best He can do with every nation on the same ground. Israel after its long sojourn in Egypt was but little removed from the heathen themselves and could not be brought to see better, so God suffered many such things and will not force men to be machines. Its care for women far surpassed and was much more merciful than that of all foreign and surrounding nations, and its mercy shows its divine origin. The laws given through Moses were light and salvation to those in darkness, leading on to still greater light.