The Lamanite king’s declaration that a war would begin was not unusual in Mesoamerican warfare. Battles could be at designated times, with warnings given. This will happen again at the beginning of the final battle at Cumorah.
At this time, Mormon moves south to the narrow neck, but still, pointedly, in the land of Desolation. They gather into a fortified city at the narrow neck which would make it harder for a Lamanite army to pass them by. They place themselves in the best possible defensive position.