Alma 43:21 - 43:25
Alma 43: 21 But they were not armed with breastplates, nor shields—therefore, they were exceedingly afraid of the armies of the Nephites because of their armor, notwithstanding their number being so much greater than the Nephites.
The Lamanites were not so well armed or protected with body armor as were the Nephites. Even thought the Lamanites greatly outnumbered the Nephites, they decided that they would suffer high causalities if they were to attacked the Nephites.
Alma 43: 22 Behold, now it came to pass that they durst not come against the Nephites in the borders of Jershon; therefore they departed out of the land of Antionum into the wilderness, and took their journey round about in the wilderness, away by the head of the river Sidon, that they might come into the land of Manti and take possession of the land; for they did not suppose that the armies of Moroni would know whither they had gone.
They therefore left the land of Antionum and traveled through the wilderness to the headwaters of the river Sidon. Their plan was to invade the land of Manti and take it. The Lamanite commanders assumed that the Nephites would not know where they would have gone in the wilderness and could take Manti with little effort.
Alma 43: 23 But it came to pass, as soon as they had departed into the wilderness Moroni sent spies into the wilderness to watch their camp; and Moroni, also, knowing of the prophecies of Alma, sent certain men unto him, desiring him that he should inquire of the Lord whither the armies of the Nephites should go to defend themselves against the Lamanites.
However, Moroni sent spies to keep tabs on the movements of the Lamanite army. In addition, Moroni sent aids to Alma for him to seek divine help in knowing just where the Lamanites were planning to attack so the Nephites could meet them.
Alma 43: 24 And it came to pass that the word of the Lord came unto Alma, and Alma informed the messengers of Moroni, that the armies of the Lamanites were marching round about in the wilderness, that they might come over into the land of Manti, that they might commence an attack upon the weaker part of the people. And those messengers went and delivered the message unto Moroni.
The Lord tells Alma that the Lamanites were traveling in the wilderness and were planning to attack the land Manit which they felt was a more venerable part of the Nephite territory and easily conquered. This message was then taken to Moroni.
Alma 43: 25 Now Moroni, leaving a part of his army in the land of Jershon, lest by any means a part of the Lamanites should come into that land and take possession of the city, took the remaining part of his army and marched over into the land of Manti.
Moroni then takes the major portion of the army to Manti to meet the Lamanites. A small contingent was left in Jershon in case the Lamanites might try to come back and try again to take Jershon.