Verse 37 tells us that the “work of death commenced on both sides.” Mormon leaves out some of the detail, but we can fill it in from this comment and the general location of the armies. The Lamanite army has moved into the valley, and must cross the river Sidon to reach their objective of Manti. The Nephite army has been divided so that part of the army is on the far side of the river, and part is on the near side. As the Lamanites move up through the valley, they pass the Nephite force that is hidden on the south side of the hill Riplah. Thus they have unknowingly placed an enemy force at their rear.
Moroni apparently waits for the Lamanite army to begin the river crossing. The Lamanite army facing Lehi’s forces at their rear will defend their rear, but still intend to move to their target. Thus they will begin to move across the river. A river fording slows down an army while they are in the water, and certainly as they come out of the water and up the banks. They will also not have as firm a battle front as if they had begun on dry land. In this state of confusion, Moroni will fall upon them with the force that was secreted on the west side of the river. This portion of Moroni’s army would not have to face the entire Lamanite force, because many of them would be on the other side of the ford. Thus the army on the west has a tactical advantage in both higher ground (the river banks must be lower than the surrounding lands) and in the ability to create a wider front.
The Lamanite army is now actually fighting on three fronts. The first is on the west, then second on the east with the rear of their forces, and the third is in the center, where the river becomes an enemy in an of itself because it hinders the rapid deployment of troops between the two fronts. The Lamanite army has been divided and surrounded, and Moroni has used the river itself as one of his weapons.