Moroni takes advantage of his advance information to lay a trap for the Lamanite armies. The terrain is key to both the trap and the nature of the action. Zarahemla, the ultimate target, lies downriver on the west side of the Sidon. Moroni places his army in a valley near the entrance to the west side of the Sidon River Valley.
Although the Lamanites clearly had a head start, and Moroni had to wait for messengers to go and return from Alma, he had arrived first. This sequence suggests that Moroni’s line of march from Jershon to Manti was much shorter, precisely what we would expect because the Lamanite army had been on the east side of the mountains which bordered the Sidon River Valley. Therefore, the Lamanite army has to travel around the mountains (and wilderness) where Moroni was able to take a more direct course up the Sidon River Valley itself.