An example. I've quoted the passage it's referring to for convenience.
1 And the LORD spake unto Moses, saying,
2 Avenge the children of Israel of the Midianites: afterward shalt thou be gathered unto thy people...
3 And Moses spake unto the people, saying, Arm some of yourselves unto the war, and let them go against the Midianites, and avenge the LORD of Midian.
4 Of every tribe a thousand, throughout all the tribes of Israel, shall ye send to the war.
5 So there were delivered out of the thousands of Israel, a thousand of every tribe, twelve thousand armed for war.
6 And Moses sent them to the war, a thousand of every tribe, them and Phinehas the son of Eleazar the priest, to the war, with the holy instruments, and the trumpets to blow in his hand.
7 And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.
8 And they slew the kings of Midian, beside the rest of them that were slain; namely, Evi, and Rekem, and Zur, and Hur, and Reba, five kings of Midian: Balaam also the son of Beor they slew with the sword.
9 And the children of Israel took all the women of Midian captives, and their little ones, and took the spoil of all their cattle, and all their flocks, and all their goods.
10 And they burnt all their cities wherein they dwelt, and all their goodly castles, with fire.
11 And they took all the spoil, and all the prey, both of men and of beasts.
12 And they brought the captives, and the prey, and the spoil, unto Moses, and Eleazar the priest, and unto the congregation of the children of Israel, unto the camp at the plains of Moab, which are by Jordan near Jericho.
13 And Moses, and Eleazar the priest, and all the princes of the congregation, went forth to meet them without the camp.
14 And Moses was wroth with the officers of the host, with the captains over thousands, and captains over hundreds, which came from the battle.
15 And Moses said unto them, Have ye saved all the women alive?
16 Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against the LORD in the matter of Peor, and there was a plague among the congregation of the LORD.
17 Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.
18 But all the women children, that have not known a man by lying with him, keep alive for yourselves.
Via the Friendly Atheist.