Alma 50:6 - 50:10
Alma 50: 6 Thus Moroni did prepare strongholds against the coming of their enemies, round about every city in all the land.
Moroni had now created strongholds to be ready for the Lamanites who were sure to come again.
I am going to assume that the people were not as righteous as they should have been or the Lamanites would not be a threat to them. The Lord had told them that they would only be troubled by the Lamanites if they were a wicked people. My feeling is that they were not especially righteous and the Lamanites thus became a real danger to the Nephites. But in spite of that, Moroni did the best he could in trying to defend against the Lamanites. I’m sure he knew that the real enemy was themselves not the Lamanites. The Lamanites were just a symptom of the disease.
Alma 50: 7 And it came to pass that Moroni caused that his armies should go forth into the east wilderness; yea, and they went forth and drove all the Lamanites who were in the east wilderness into their own lands, which were south of the land of Zarahemla.
Moroni’s next step was to drive out all the Lamanites living in the east wilderness back to their own country the land of Nephi to the south.
I guess he was trying to create a buffer zone between the Nephites and the Lamanites besides the fortifications. Something the Lamanites would have to cross without any help from Lamanites already living there.
Alma 50: 8 And the land of Nephi did run in a straight course from the east sea to the west.
The area under Nephite control was a straight sweep of land that ran east and west starting from the east sea.
Alma 50: 9 And it came to pass that when Moroni had driven all the Lamanites out of the east wilderness, which was north of the lands of their own possessions, he caused that the inhabitants who were in the land of Zarahemla and in the land round about should go forth into the east wilderness, even to the borders by the seashore, and possess the land.
Now that the Lamanites were driven out of the east wilderness which ran all the way to the east sea and which was just north of the lands of the Nephites, Moroni caused the Nephites from the area around Zarahemla to go in the wilderness and take control of the area to keep it from falling into the hands of the Lamanites.
Alma 50: 10 And he also placed armies on the south, in the borders of their possessions, and caused them to erect fortifications that they might secure their armies and their people from the hands of their enemies.
Moroni also built fortifications on the south of Nephite lands to guard the southern borders against the Lamanites.