There is another event that occurs in this year, but it was unrelated to specific military actions. Therefore, Mormon introduces this new story, with a rather abrupt transition from a new threat near the west sea, south, about the people of Ammon. It has been a while since Mormon has mentioned them, and so he recapitulates enough of their story that his readers will know for certain the essential details that lay behind it.
The Ammonites were known as the Anti-Nephi-Lehies when they lived in Lamanites lands, and they immigrated to Nephite lands and were originally given Jershon as a land for their inheritance. These were the people who had taken an oath not to take up arms. This story will develop the effect that oath had upon the Nephites.
The first important part of the story was that the Nephites knew and accepted the oath they had taken, and therefore, the Nephites protected them (see Alma 27:24). The defection of the Zoramites in Antionum opened a direct threat against the Ammonites, so they had to be moved to Melek (see Alma 35:13).