Numbers 31:1,2

And the Lord spake unto Moses, saying— After Moses had repeated to the people the laws in the foregoing Chapter s, God determined, before he took this great lawgiver to himself, to use his ministration for the punishment of the Midianites, who had so abominably seduced the Israelites to wickedness. The Moabites, though guilty also, were now spared; some circumstances which the sacred history has suppressed, no doubt, occasioned this distinction. Probably, the Midianites contributed more than the Moabites to the seduction of the Hebrews. See Parker's occasional Annotation on the chapter.