In the previous war, the Lamanites had blitzed through the Nephite countryside, driving the Nephites before them. Now, however, thanks to Mormon’s decade of preparations, the fortifications and, probably, fertile northern areas, the Nephites withstood the attack.
They cast the Lamanite dead into the Sidon, which leads to the sea. They had taken this same course in disposing of the dead in the first Moroni’s wars. (See commentary accompanying Alma 3:3.)