In this Parsha G-d commands Moshe to take vengeance for the Children of Israel against the Midianites. The commentators are all puzzled by the fact that Moshe is not commanded to take vengeance against the Moabites when it was the Moabite women who seduced the Jewish men into intimate immorality and idolatry.
Abarbanel explains that the Midianites were actually the instigators. He explains that Bilaam, after having failed to curse the Children of Israel, passed through Midian and advised them that the only way to bring about the downfall of the Jewish nation was through seduction. Bilaam remained in Midian in order to work out a plan. The Midianites gave their daughters free rein to encourage Klal Yisrael to transgress. Since the Jewish People had no quarrel with the Moabites, and had actually been instructed not to distress them or provoke war with them, the Midianite women were able to disguise themselves as Moabites and were able to circulate among the Jews, ostensibly selling them food and other supplies. Thus, when the Torah states in Parshat Balak, “Israel settled in Sheetim and the people began to commit harlotry with the daughters of Moab” — they were actually involved with Midianite women.
Abarbanel offers another possible explanation. When the Torah states that “the people began to commit harlotry” it is referring to the idea that the Moabite women acted first, and when the Midianite women saw that the men were vulnerable they decided to use such seduction as a means to entice them to idolatry as well. This is why taking vengeance referred to Midian alone, as the Torah states in Parshat Pinchas, “Harass the Midianites and smite them, for they harassed you through their conspiracy that they conspired against you in the matter of Peor (the idol the Jews were enticed to worship).” The Moabite women caused the men to give in to their physical desires, but the Midianite women enticed them to commit idolatry, a much more serious and damaging transgression.
According to both explanations, however, the Midianite women were clearly guilty of bringing about both immorality and idolatry. This is reflected in Moshe’s charge to the people after G-d had instructed him to “take vengeance for the Jewish People against the Midianites.” When he instructs the people, however, he tells them: “Arm men from among yourselves, that they may be against Midian to inflict G-d’s vengeance against Midian.” The vengeance for intimate immorality belongs to the people, but the vengeance for idolatry belongs to G-d.