Lehi’s attack from the rear no doubt caused confusion in the center since the Lamanites would have pressed more hurriedly across the river. These are the same warriors who had been engaged with Lehi in an apparently losing cause. They assume that relative safety might lie across the river. However, they are attacked from the front as they come up the bank. As they moved on Manti they were probably less well formed into a combat unit. Thus, when they got closer to Manti and met what was probably a larger force than those directed by Moroni and Lehi, they would have been more tired, still confused, and much more poorly organized. The Manti army, under an unnamed commander, would have had a great advantage, even though I speculate that these would be local fighters and not part of Moroni’s more well-trained units.
Moroni has two types of troops: (1) his own army, marched up from Jershon and known to be wearing protective clothing, and (2) the local Manti militia, probably less well-equipped and less well-trained, though fresher. Moroni matched these forces to the specific engagements within the larger battle. He would have left most of his seasoned troops (with protective armor) with Lehi on the east side of the river. He took a smaller number of these troops with him to the west side of the river in a valley.
He left the largest number of warriors, but least trained, in defense of Manti. This unit might not have been the equal of the Lamanites man to man, but Moroni had made sure that the Lamanite army had been softened up by their first encounter, thus enhancing the effectiveness of his militia.