The Lord told Moses in yesterday's passage that he would see Israel avenged upon the Midianites before he dies. Moses now relays the Lord's battle plans to Israel.
"So Moses said to the people, 'Arm some of your men to go to war against the Midianites so that they may carry out the Lord's vengeance on them. Send into battle a thousand men from each of the tribes of Israel.' So twelve thousand men armed for battle, a thousand from each tribe, were supplied from the clans of Israel. Moses sent them into battle, a thousand from each tribe, along with Phinehas son of Eleazar, the priest, who took with him articles from the sanctuary and the trumpets for signaling." (Numbers 31:3-6) These are the trumpets the Lord commanded to be fashioned out of silver in Numbers 10. They were to be blown only by the priests and they were used for several purposes: when both were sounded in the camp, all the Israelites were to assemble; if only one was sounded in the camp, only the leaders were to assemble. Also, whenever the Lord told the Israelites to break camp, trumpet blasts were used to signal when each tribe was to move out. The trumpets were to be blown at festivals and times of rejoicing. And finally, as in our passage today, the trumpets were blown before going into battle: "Then you will be remembered by the Lord your God and rescued from your enemies." (Numbers 10:9b)
"They fought against Midian, as the Lord commanded Moses, and killed every man." (Numbers 31:7) Every Midianite male who was in the region and who engaged in battle with the Israelites was killed, not every Midianite male who existed. We find the Midianites still making themselves the enemy of Israel on up through the book of Judges, so it goes without saying that the Israelites do not encounter 100% of the male Midianites here in Numbers 31.
The Midianites are related to the Israelites by marriage through Moses: his late wife was a Midianite. Moses' children are half Midianite. Moses' father-in-law was a Midianite priest who served the Lord, so it's not as if the Midianites weren't aware of the one true God whom the Israelites worship. At some time in the past, every human being on the face of the earth knew about the one true God, but by this point in the Bible much of the Midianite population has turned away from Him to serve heathen idols. We see that the Midianites and the Israelites had some things in common---things which should have encouraged the Midianites to extend the hand of friendship---but when the Israelites camped nearby on their way to the promised land, the Midianites feared them for no reason. The Midianites harbored prejudice against them for no reason. Due to the familial connection by marriage, the Midianites should have sent ambassadors to greet Moses and the leaders of Israel. If they were concerned about Israel's intentions toward them, it would have been a simple matter to find out what Israel was doing and where Israel was going. But the Midianites are wicked, and wicked people tend to assume others are as wicked as they are. Wicked people tend to be suspicious, which is why the Bible says, "The wicked flee when no one pursues," (Proverbs 28:1a) and that the wicked imagine conspiracies where no conspiracies exist and fall into unnecessary panic as a result (Isaiah 8:12).
The 12,000 Israelite soldiers kill every Midianite soldier with whom they engage in battle, including some important political figures who are mentioned by name. "Among their victims were Evi, Rekem, Zur, Hur and Reba---the five kings of Midian." (Number 31:8a) In other passages of the Bible these men are referred to as dukes or princes. I looked this verse up in a Hebrew concordance and found that the word translated as "kings" can also be translated as "royalty". I believe these men were tribal chiefs because the one known as Zur is likely the same Zur from Numbers 31 whose daughter Kozbi seduced an Israelite man named Zimri into idolatry and strode boldly into the Israelite camp with him to have sexual relations with him in his tent. We were told in Chapter 25 that Zur was a tribal chief of a Midianite family and I tend to think the five men named here in verse 8 were all tribal chiefs. It appears the land of Midian was divided into five regions and each of these men was head of a region.
Another important figure was killed at this time by the Israelite soldiers. "They also killed Balaam son of Beor with the sword." (Numbers 31:8b) The prophet who was willing to curse Israel in exchange for fortune and fame loses his life. The man who didn't care if an entire race of people was wiped off the face of the earth is himself wiped off the face of the earth. Previously we were told Balaam returned to his hometown but many scholars believe that, when the Midianites saw that war with Israel was unavoidable and imminent, they called the prophet back to see if there was anything he could do to weaken Israel and to give Israel's enemies success. What a sad end to a man who was once a true prophet of the Lord! What a terrible legacy to leave behind: to be known as a man who forsook the Lord and the eternal blessings that come from Him in exchange for worldly wealth and renown. Balaam wanted his name known---and it is!---but in a very negative way. He's mentioned on the pages of the Bible, not as an example to follow, but as an example of what not to do.
As was the custom of the day, the winning side takes the people and the livestock and the belongings of the vanquished foe as the spoils of war. "The Israelites captured the Midianite women and took all the Midianite herds, flocks and goods as plunder. They burned all the towns where the Midianites had settled, as well as all their camps. They took all the plunder and spoils, including the people and animals, and brought the captives, spoils and plunder to Moses and Eleazar the priest and the Israelite assembly at their camp on the plains of Moab, by the Jordan across from Jericho." (Numbers 31:9-12)
When we conclude Chapter 31 tomorrow we'll find Moses angry about part of these spoils of war. It appears a particular command he gave to the soldiers was not followed. Moses will ensure that this oversight is remedied. We may find tomorrow's passage disturbing but at the same time I think we can come to terms with it by considering it from the Lord's perspective and considering how not following this particular command could have led to a great deal of harm coming to Israel in the future.