For Mormon, the disadvantaged military position of the Nephites would have highlighted their reliance upon the Lord for their salvation. Just as Mormon discounted the Nephites' advantaged position in the battle with the Amlicites, here is emphasizes their disadvantaged position to highlight the necessity of their reliance upon the Lord as the true source of their success.
Redaction: This verse is a summary of the day, and is interestingly placed prior to the description of the events that lead to the defeat of the Lamanites. Mormon's point is that the Lord preserved the victory. Since that is his most crucial point, he states it first, and then shows how it was true.