15 - 21 Coriantumr is a really big and really smart dude. the king of the lamanites figures he can send Coriantumr in and he will totally wipe the floor with the nephites.
23-29 Coriantumr figures he has the nephites on the run so he just keeps plowing the road. the leader of the nephites sees this and decides he better nip this thing in the bud. lehi and his army are sent to head off the lamanites and succeed in keeping them from doing further damage. whew! that was close!