Alma 53:8

And now it came to pass that the armies of the Lamanites, on the West Sea, south, while in the absence of Moroni, on account of some intrigue amongst the Nephites, which caused dissensions amongst them, had gained some ground over the Nephites, yea, insomuch that they had obtained possession of a number of their cities in that part of the land;

The Lamanite armies were on the West Sea to the south. Moroni was not there. Some Nephites were making secret plans. This caused fights among the Nephites. The Lamanites gained ground over the Nephites. The Lamanites took over some of their cities in that part of the land.

Meanwhile, the Lamanite armies on the southern coast of the West Sea had gained ground during Moroni’s absence. Some scheming and betrayal among the Nephites had caused divisions, and the Lamanites had exploited the chaos to capture several cities in that region.