Numbers 21:12–25:5 compromise three independent stories that when placed one after the other produce some noticeable narrative inconsistencies, especially when speaking about the presence or lack thereof of Moabites in the area.
1) Numbers 21:12-35, which I’ve already treated in contradictions #281, #282-285, and #286 comes from the Yahwist tradition and narrates the Israelite conquest of Amorite territory in Transjordan north of the Arnon river. All of this territory is presented as Amorite, and there are surprisingly no references to Moabites other than the mention of their defeat and utter annihilation a long time ago by the Amorite king Sihon—a conquest preserved in the so-called Heshbon Ballad:
Woe unto you Moab: you’ve perished, Chemosh’s people. He has made his sons refugees and his daughters captive to the king of the Amorite, Sihon.
Why northern Moab is here portrayed as Amorite territory in the Yahwist source, and what the intentions were of its author who portrayed the Israelites taking this territory from the Amorites and not from the Moabites has already been discussed in #271-273 & #382-285.
2) Numbers 22-24, the Balaam pericope, however, now presents a Moabite force and a Moabite king! They appear on the stage from nowhere. Readers have long inferred that the Balaam story comes from a separate and once independent tradition. It was most likely placed in its present context due to the geographical proximity with the previous story. However, contrary to the preceding Yahwist material, where the Israelites end up settling in Amorite territory north of the Arnon and even further north in Bashan, this story suddenly places them on the border of Moabite territory and threatened by a sizable Moabite presence in the land.
3) And lastly, the brief episode of the Baal Peor sin in Numbers 25:1-5, where the Moabites are generously represented as well, actually reflects the religious syncretism in Transjordan of a later time period, and has nothing to do with the preceding two stories. It too was placed here due to its geographical proximity with the previous episodes.
We will look at each one of these stories in more detail over the forthcoming entries.
Here is an excellent article by Dr. Rachel Havrelock:
http://thetorah.com/inventing-the-mythic-amorite-kingdom-of-sihon/