Midianites are blamed for Baal Peor and for the harlotry. In Bemidbar 25:16-18:
Moav to the daughters of Midian?
One possibility is that both Midian and Moav were involved, but so long as Moav took no physical belligerent step, they were immune from attack. After all, we read in Devarim 2:8-11:
Bemidbar 31:8), and that this enticement was at the advice of Bilaam ben Beor (Bemidbar 31:16).
Earlier, in parshat Balak, we see the Midianites involved in the consulting of Bilaam, and that they were among the princes sent to Bilaam. In Bemidbar 22:4:
I do not know what the political situation situation was on the ground with regard to Midian and Moav, but from parshat Balak, it seems the rulers had some connection. We know they contended earlier. In the lists of Edomite kings, we read:
to the elders of Midian But did they not always hate each other, as it says, “who defeated Midian in the field of Moab” (Gen. 36:35), when Midian came against Moab in battle? However, because of their mutual fear of Israel they made peace with each other. And what did Moab see to take counsel with Midian? Since they saw that Israel was supernaturally victorious [in their battles], they said, “The leader of these [people] was raised in Midian. Let us ask them what his character is.” They told them, “His strength is solely in his mouth.” They said,“We too will come against them with a man whose strength is in his mouth.” - [Mid. Tanchuma Balak 3, Num. Rabbah 20:4]According to this, Moav fought at times with Midian, but here made peace. Alternatively, this is Edom fighting Midian, in the plains of Moav. This would mean that Midian rules over some Moabite territory - the field of Moav. This would make sense, as we see that these countries wages wars against each other, taking over territory. Thus, for example, the land of Sichon had been taken over from the children of Ammon, as discussed previously.
Thus, we might expect there to be Moabite women and Midianite women living in close proximity. Indeed, there might be Moabite commoners and Midianite nobles living in this land. We would expect commoners to go with commoners, and Israelite nobles, such as Zimri ben Salu, to take noble women. Indeed, this is part of the import of the giving of the lineage of both Zimri and Kozbi. Bemidbar 25:14-15:
at that time He was not entitled to the monarchy. He was one of the Midianite nobles [according to some: of the nobles of Sihon (Josh. 13:21)], and when Sihon died, they appointed him over them on a temporary basis. — [Mid. Tanchuma Balak 4, Num. Rabbah 20:4]the Midrash is picking up on the strange phrase that he was king of Moav at that time. On a pshat level it is just identifying the king of Moav during this episode, but the Midrash takes this as an asterisk on Balak's record.
If Midianites and Moabites were having their their daughters do this, but the location was initially that of Moav, or the commoners were Moabites, or the majority of participants were Moabites, or it was a Moabite area ruled by nobles from Midian, then we can see why the pasuk initially describes the enticers as the daughters of Moav.