Charles Ellicott Commentary Numbers 31:18

"But all the women-children, that have not known man by lying with him, keep alive for yourselves." — Numbers 31:18 (ASV)

Keep alive for yourselves. —The Israelites were allowed to make slaves of their captives. Shortly after the capture of these Midianitish women, and, perhaps, as a result of it, the law concerning marriage with captives was enacted. (See Deuteronomy 21:10-14.)