Verses
Alma 24:14 Now the king (Ammoron) had departed out of the land of Zarahemla, and had made known unto the queen concerning the death of his brother, and had gathered together a large number of men, and had marched forth against the Nephites, on the borders by the west sea;
Alma 24:15 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.
Alma 24:17 But behold, it came to pass in the twenty and seventh year of the reign of the Judges, that Teancum, by the command of Moroni, who had established armies to protect the south and west borders of the land, had begun his march towards the land Bountiful, that he might assist Teancum with his men, in retaking the cities which they had lost.
Commentary: Moroni, not Teancum, left the south and west borders of the land. Teancum could not be in two places at one time. In Alma 24:21, it's mentioned, "And it came to pass that Moroni did arrive with his army to the land of Bountiful, in the latter end of the twenty and seventh year of the reign of the Judges over the people of Nephi."