The fact that the Lord has told Moroni that the main attack will be at Manti does not preclude some smaller attack at Jershon. Moroni therefore leaves some forces behind to protect that very vulnerable land.
Variant: Editions subsequent to the first edition have eliminated a second reference to Moroni in this passage. The original read: “take possession of the city, and Moroni took the remaining…” The “and Moroni” was also present in the original and printer’s manuscripts. This change was made to smooth out the sentence structure when the phrases were seen as a single sentence. The original manuscript was not neatly broken into sentences, and an ancient text would not have had sentence punctuation either. The “and Moroni” is a marker of a new idea, a new sentence. That function has been lost by the removal, but no real information.