Deuteronomy 21:11

And hast a desire unto her Moses here returning to the case of war with the neighbouring nations, directs that, if a Hebrew soldier conceived a peculiar regard for a captive woman, and desired to marry her, he must not do it immediately after she became his prisoner, it being of dangerous consequence for the Israelites to marry Gentile wives. He was first to keep the woman in his house for a month, at least, where she was to live in the retirement and habit of a mourner, for the loss of her parents and her country; as also to give her time to be instructed in the knowledge of the true God and his will, and renounce her idolatrous worship, and to allow him sufficient space to try whether his affection for her was calm and steady, or might cool and wear off. If this interval caused no abatement of his love, but, upon her turning proselyte, he still desired to make her his wife, he might then lawfully do it.