The Midianites
The people of Midian together with Moab began to interact with the people of Israel. The Israelites were staying in Shittim and the Israelite men began to commit whoredom with the Moabite women. The Moabite women invited the men to worship the Moabite gods. The LORD God of Israel then ordered a daylight execution of all those that had transgressed by fornication and idolatry with the Moabites, with the cooperation of the Moabite leaders. After this decree, an Israelite man, Zimri the son of Salu, publicly brought a Midianite woman, Cozbi daughter of Zur, into his tent. Phinehas son of Eleazar, left the congregation, went into the tents inner chamber, and drove a javelin through both Zimri and Cozbi. Because of Phinehas' act, the LORD's wrath was turned away from the children of Israel. The LORD then ordered Moses to vex the Midianites and smite them, for plotting against the Israelites in the matters of Peor and Cozbi.
When a 12,000 men army (1,000 from each tribe) was sent to "avenge the LORD of Midian", Moses was angry ("wroth") when the army returned with captive women and other spoils, because the women had played vital roles in the previous trespasses and consequent plague.
Moses then ordered all males and non-virgin females killed, and all female virgins preserved. One of every five hundred remaining captives of the soldiers' plunder was to be put in service as tribute to the LORD under Eleazar the priest (Numbers 31:28) as "heave offerings", thanksgiving for success. Out of the booty that belonged to Israelites, one of every fifty was set apart to the Levites who cared for the Tabernacle. (Numbers 31:30)
Read more about this topic: Crime And Punishment In The Bible