The Reader will not forget to observe that Midian, and not Moab, was first the seat of war; though it is probable, from what we read before, of Midian and Moab acting in conjunction, to get Balaam, the pretended conjuror, to curse Israel, that both nations were in colleague. See Numbers 22:7.
Related Commentaries of Numbers 31:7
Numbers 31:7
7 And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.