Textual: This verse serves as a transition into a new literary unit. Mormon is closing the flashback portion of the text, and now begins new material. That new material will be in a separate chapter, befitting the beginning of the return to the main story line.
“They Could Find No Way to Deliver Themselves”
There is no way out of their bondage except to flee. They have tried military might three times, and each has resulted in defeat and the dreadful loss of men. The city now finds itself with a smaller population, and a much-diminished population of fighting men. Military might is simply no longer an option. There is nothing to negotiate, and all negotiation favors the Lamanites. There is only escape.