Mormon had declined to lead the Nephite armies because they had begun to believe that they were strong enough to go on the attack. Without Mormon to attempt to deter them, they did attack, leaving their stronghold and moving into the land southward.
Mormon had been correct. They were defeated, and driven back to the point where they lost an important defensive city near the narrow neck. At this point in his story, Mormon is repeatedly describing the Nephites in the land of Desolation, and even that they had a city called Desolation. He is declaring that the Nephites are now in both the physical and spiritual place that caused the destruction of the Jaredites, and which will lead to their own destruction.