Part of the military plan used the river itself as part of the defense. Once the Lamanite army was in the river, they were less mobile by the very nature of crossing through water. Thus, Lehi, the Nephite army’s leader, waits until the Lamanites have begun crossing, and then institutes a surprise attack on their rear. This attack from the rear not only would cause the Lamanites to have to rapidly reorient themselves, but it also would delay some of the Lamanite army from coming against the Nephites precisely because they were already in the river.