Alma 50
It believe that some of the new fortifications that Moroni built were based on the experience of the fist attack by the army of Amalikiah. All his men were alive, but many of th 50 who were wounded were seriously hurt in their legs – thus he built timberworks that would protect them plus additional towers etc. Having fresh evidence that the Lamanites were willing to attack, Moroni took the initiative to fortify their lands by driving out the Lamanites that had been living in the borders of Nephite territory.
While we are shown that the people were prosperous I am confident that they did not feel particularly prosperous. They probably felt as if they were in a perpetual state of alert and danger (they were) and it was difficult to feel comfortable with the great material prosperity that they enjoyed as they built new cities and fortified their lands. Perhaps that is the only way to be truly prosperous is to be under a certain amount of stress. That stress over year may be what brought the people of Mroianton to make the poor choice to contend with their neighbors.
I forgot that Alma had apparently offered the sacerd records to Nephihah, the chief judge, before conferring them on Helaman, his son.