Alma 51:22

Behold, it came to pass that while Moroni was thus breaking down the wars and contentions among his own people, and subjecting them to peace and civilization, and making regulations to prepare for war against the Lamanites, behold, the Lamanites had come into the land of Moroni, which was in the borders by the seashore.

While Moroni was stopping the wars and fights among his own people and making peace, and getting ready for war against the Lamanites, the Lamanites had come into the land of Moroni. This was on the border by the ocean.

While Moroni was crushing the civil war among his own people, restoring peace and order, and making preparations to fight the Lamanites, the Lamanites had invaded the city of Moroni on the coast.