Once again the Lamanites allowed themselves to be lulled by their intelligence. Having determined that the path of march of the Nephite columns was toward Zarahemla, they stop. Both armies had evidently marched for a fair distance, as the Lamanites make camp rather than return to Manti. They were far enough away, and had a large enough force, that it was more prudent to stop for the night.
Helaman has his troops perform a dangerous forced night march. By acting while their enemy slept, they were able to return to Manti and find it in Nephite hands.