The “war began to be among them in the borders of Zarahemla, by the waters of Sidon.” This would be upriver from Zarahemla proper, and closer to the location of Manti (if that city still stood;that would be the most typical approach point of the Lamanites from the south. The geography dictated that it was the natural entry point, and that has not changed in the over three hundred years since the last reported war.
At this point, we will begin to see more strength numbers listed for military units. Mormon was a military man, and also a military historian. Thus, the sizes of the armies were important to him. Nevertheless, this was an era where tallying large numbers would have been difficult and there was no science of calculating population numbers. Thus, we will see numbers, but as with most ancient records, the large numbers may not have represented the actual number, which was likely smaller.
As with other times in the past, the Nephites are victorious, and the Lamanites retreat. There is peace for four years, which is one of the longer periods of peace (excluding the two hundred years of peace as mentioned in 4 Nephi 1, of course).