Alma 43:4

For behold, it came to pass that the Zoramites became Lamanites; therefore, in the commencement of the eighteenth year, the people of the Nephites saw that the Lamanites were coming upon them; therefore they made preparations for war; yea, they gathered together their armies in the land of Jershon.

The Zoramites became Lamanites. So at the start of the eighteenth year, the Nephites saw that the Lamanites were coming to attack them. The Nephites got ready for war. They brought their armies together in the land of Jershon.

The Zoramites had joined forces with the Lamanites. When the eighteenth year began, the Nephites saw the Lamanites coming and prepared for war. They gathered their armies in the land of Jershon.