What foreign men could not do (defeat Israel by words or arms), women set out to accomplish (Num 25).  Apparently, Balaam cooked up the plot (Num 31:16) to seduce Israelite men using women of Moab and Midian. They also involved the Israelites in pagan sacrifices.   Balaam knew that what no man could take away from Israel -- the blessing of God -- Israel could forfeit through sin.   One man even took a Midianite woman into his tent in the sight of Moses  --  both the man and woman were speared to death by Phinehas the priest. This act appeased God's anger against Israel, and stopped a plague that was raging in the camp.  From this time on, the Midianites were considered enemies of Israel (Num 25:17).  As late as the time of Nehemiah after the Captivity, Ammonites and Moabites were still excluded from the assembly on the basis of hiring Balaam (Neh 13:1-3)!

         

God's judgment on the Midianites for their part in the seduction of Israel was not delayed (Num 31).  This was Moses' last assignment from God before his death.  12,000 Israelite men fought against Midian and killed 5 kings, as well as Balaam (Num 31:9).  This was a ruthless extermination campaign, a holy war, but it must be recalled that it was provoked by Midian.  Because the Midianites had caused Israel to sin against God, it was not just a matter of defeating or subjugating them  -- they were utterly destroyed.  Only virgin girls were spared from death.  All the plunder had to be purified as well, by fire or water.  

         

This strengthens the argument made above that Moses' wife in Num 12 was another woman than Zipporah, who was a Midianite, and that this second wife was really a Cushite.  Otherwise, Moses' new wife could have been declared an enemy of Israel.  How could he judge others when he himself kept a Midianite wife?  Certainly the Israelites, ever quick to complain, would have accused him of hypocrisy.