Zerahemnah had not wanted to take the oath, and he provoked others to stand and fight with him. They did. They had known that they were hopelessly outnumbered, but in individual warfare, only so many men can engage at one time. Although not stated, it is probable that the Lamanites stood back to back so that they provided a fighting front to all around them. Still, even the best of fighters must tire. Even though the Nephites could not bring all of their army to bear simultaneously, they could keep providing fresh troops where the Lamanites simply continued to exhaust their strength, even if they were individually fighting well.
There came a point when the battle was clearly over, and Zerahemnah makes it known that they are finally willing to enter into a covenant of peace. Moroni had not put a time limit on the offer of the covenant. Therefore, Moroni once again ceases the fighting. This time, the remainder of the Lamanites accept the covenant and leave.