Midrasch zu Bamidbar 31:10

(Bamidbar 31:7) "And they warred against Midian": They surrounded it from four sides. R. Nathan says: They left a fourth side for them to flee from. (Ibid. 8) "the five kings of Midian": As they were all one in counsel, they were all one in death. "and Bilam the son of Beor they slew by the sword": Israel gave him his full reward and did not stint him. For he said to them: When you were six hundred thousand (in the days of Balak), you could not withstand them, and would you withstand them now? Whereupon they gave him his full "reward" (for his sound advice) and did not "stint" him. R. Nathan says: With the four judicial death penalties they slew him. As it is written (Joshua 13:22) "And Bilam the son of Beor the sorcerer the children of Israel slew by the sword together with their slain." (Bamidbar 31:10) "and all tirotham": This refers to their houses of idolatry. Variantly: the posts of their sentinels. (Ibid. 31) "And they took all of the spoil … and they brought it to Moses." Scripture here apprises us that they were just and upright and not suspect of theft, as opposed to (Joshua 7:1) "And the children of Israel embezzled the spoils," while here "And they took all of the spoil … and they brought it to Moses." (Bamidbar 31:13) "And Moses and Elazar the Cohein went out": Abba Channan says in the name of R. Eliezer: Because they saw the youth of Israel going out to snatch the spoils. (Ibid. 14) "And Moses was wroth with the commanders of the host": "the great ones bear the stigma." — whereupon Pinchas said to him: "Our teacher, we did as you commanded us." (Ibid. 15-16) "And Moses said to them: Have you let all the females live? These are the women who were (consigned as harlots) against the children of Israel by the word of Bilam." What was that word? He said to them (the Midianites): Even if you bring all the hordes in the world against them, you will not defeat them. Are you more numerous than the Egyptians, of whom it is written (Shemot 14:7) "And he took six hundred of his picked chariots, etc."? But come, I will counsel you as to what to do. The G-d of these detests lewdness. Consign your wives and daughters to them and steep them in lust and their G-d will war against them. For this is the rule: As long as Israel does His will, He wars for them, as it is written (Shemot 14:14) "the L-rd will war for you." And when they do not do His will, He wars against them, viz. (Isaiah 63:10) "… and He turned into their foe." And, what is more, the Merciful One becomes cruel to them, viz. (Eichah 2:5) "The L-rd has become like a foe; He has swallowed up Israel."