This is the first battle of the conquest. We know that Israel sent 12,000 men into combat. We know 32,000 virgin girls were taken as spoil (dare I say bootie?). If we assume the girls were 1/3 of the Midianite females, that would be a total population of about 180,000. If the Midianite Army was about 1/3 of the males, that would again be about 30,000 (the same as the girls). All the Midianite men (maybe 30,000 in the Army and another 30,000 that were too old) were killed in the battle with ZERO Israelite casualties! This is an amazing trend that will continue.
But then Moses ordered the Israelites to kill all the (Non combatant) boys and women. Is that something God wanted? This battle reinforces God's instructions to destroy the Canaanite cultures. These peoples had gone so far into the pagan worship of Baal, Asherah, and Moleck, to include ritualistic immoral sexual practices and child sacrifice, that their sinful ideas had corrupted them too far to be redeemed (at this time). These ideas present a threat to the Jewish culture which God built in the 400 yrs in Egypt and Moses has struggled to keep for the last 40 years in the desert. These cultures and their poisonous, evil, anti-Yahweh ideas have to be wiped clean for Israel to survive. The corruption against God must be eliminated and He promised not to do the flood thing again. This is about ideas, not a people, but the people who have these ideas (grouped as a culture) cannot be saved before the coming of the Christ.