Then they went through the wilderness, and went around the land of Edom, and the land of Moab, and came by the east side of the land of Moab, and they encamped on the other side of the Arnon; but they didn't come within the border of Moab, for the Arnon was the border of Moab.
Second, he emphasized the fact that Yahweh had given Israel this land. Thus it would have been wrong theologically to allow the Ammonites to take it from them (Judges 11:23-25).