And Moav said to the elders of Midyan: Now the assembly will lick up all that is around us, as the ox licks up the greenery of the field. And Balak the son of Tzippor was king of Moav at that time.
Bamidbar Rabbah 20:4 — why Moav consulted Midyan; Bilam’s homeland.
Sefaria — Bamidbar Rabbah 20:4
English: “And Moav said to the elders of Midyan.” Why did they come to Midyan? Because they saw that Israel was victorious not in the ordinary way of the world; they said: The leader of these was raised in Midyan — let us ask them what his measure is.
Narrative cause for the Moav–Midyan consultation.
The alliance begins from recognition that Moshe’s strength is not conventional war — Midyan is consulted because that is where he was raised.