Even though the Lamanites had entered into a treaty to give the land northward to the Nephites, they secretly planned to take over the whole western hemisphere. The Lamanites took ten years to prepare themselves for this mammoth conquest.
It is obvious that Mormon had no expectation that the Lamanites would live up to their treaty. He knew their greed would never be actually satisfied, and fully expected that eventually the Lamanites would launch a massive attack against the land northward. Meanwhile he tried to prepare the people with arms and forts to meet the inevitable assault.