In Moroni 9:2, Mormon reported a battle in which the Nephites ultimately did not conquer. This may have been the result of several earlier skirmishes in which Mormon was personally involved, but this time it seems to have been a major turning point. In Mormon 2:19 the Nephites had been routed temporally, but in 2:25 they regrouped and stood their ground and in 2:27 they retook their lands of inheritance. In Mormon 4:13, for the first time, there will be no such recovery.
The fact that Mormon says here “I have had a sore battle with the Lamanites in which we did not conquer” is admittedly an important statement. It would mean that Moroni 9 was not written between AD 362 and 375, during the period when Mormon was not serving as the leader, unless it is possible that Mormon was involved in the battle mentioned in Moroni 9:2 in some way other than as the commander or leader. But, more likely, that battle in which the Nephites “did not conquer” occurred at the time when Mormon had just begun again as commander and the Nephites were forced to flee from the battlefield, taking refuge in the city of Jordan (Mormon 5:3, about AD 376).
Mentioning over a dozen specific points or words found in Mormon 4, the letter in Moroni 9 then goes on to describe the awful depths to which the Nephites had sunk during the decade from AD 366 to 375. Recapping that deterioration of Nephite faith, civility, and worthiness before God, Moroni 9 sets forth Mormon’s justifications for his despondent assessment of the hopeless condition of the Nephites around AD 375–376.
John W. Welch, “Mormon 1–6,” in John W. Welch Notes (Springville, UT: Book of Mormon Central, 2020), 1059–1060.