The Lamanite/Gadianton army is employing a scorched-earth policy, razing and burning the cities they take. This tactic removes usable land and resources from both armies. It is effective for an attacking army, if it can move swiftly enough or if it has an assured supply line. Apparently, the Lamanites were capable of achieving both.
Chronology: Three hundred and seventy nine years would be A.D. 370. Mormon is now about sixty-nine.