When the combined armies of the Amlicites and Lamanites are forced across the river Sidon, they appear to understand that they have been defeated, and, therefore, they begin to flee the battlefield. The miracle of the victory is emphasized when Mormon notes that those who were defeated “were so numerous that they could not be numbered.”
The combined Amlicite and Lamanite armies flee into the wilderness to the northwest of the Nephite lands, even though Lamanites typically enter Nephite lands from the south. This suggests that they were fleeing, and that their typical means of entrance was blocked by the Nephite armies.
We do not hear of many incursions from the west, and these verses help readers understand why. This was a dangerous wilderness. Mormon declares that many who fled in that direction did not survive, but fell to the beasts of the wilderness.
Although our current chapter ends here, Mormon’s original chapter continues without a break. The story of the war is over, but Mormon originally included the aftermath as part of his whole chapter. That story now comes in the next chapter.