Just after we read of Moroni notifying Teancum of Lamanites which were upon the Nephites in the borders of the land by the west sea, we find an editorial note by Mormon in which he states:
Now the king [of the Lamanites--Ammoron] had departed out of the land of Zarahemla, and had made known unto the queen concerning the death of his brother [Amalickiah], and had gathered together a large number of men, and had marched forth against the Nephites on the borders by the west sea. And thus he was endeavoring to harass the Nephites, and to draw away a part of their forces to that part of the land, while he had commanded those whom he had left to possess the cities which he had taken, that they should also harass the Nephites on the borders by the east sea, and should take possession of their lands as much as it was in their power, according to the power of their armies. And thus were the Nephites in those dangerous circumstances in the ending of the twenty and sixth year of the reign of the judges over the people of Nephi. (Alma 52:12-14)
The reader should be aware that the whole story of these battles by the west sea will be told in Alma 53:8-18 and Alma 56:1--58:39. [Alan C. Miner, Personal Notes]