Jershon was a land the Nephites gave to the people of Ammon, the converted Lamanites also called Anti-Nephi-Lehies, for an inheritance. It lay on the east by the sea, joining and south of the land Bountiful (Alma 27:22). The converts had renounced war and sworn never to take up arms, so on admitting them the Nephites set their armies between Jershon and the land of Nephi to guard them, in exchange for a portion of the converts’ substance to maintain those armies (Alma 27:23-24). The people went down and took possession of Jershon, and were called the people of Ammon thereafter (Alma 27:26).
Once they were established and a church set up in the land, the Lamanite armies that had followed them into the wilderness attacked, in a battle in which tens of thousands of Lamanites were slain alongside heavy Nephite losses (Alma 28:1-3). The dead were too many to number (Alma 30:2). Ammon served as high priest over the people there. When Korihor came to Jershon to preach, the people seized him, bound him, and carried him before Ammon, who had him taken out of the land (Alma 30:19-21).
After the mission to the Zoramites, those who accepted Alma’s preaching were cast out of Antionum and came to Jershon, where the people of Ammon received the poor among them, clothed and nourished them, and gave them lands (Alma 35:6, 9). The Zoramite leaders, angered, allied with the Lamanites and pressed toward war (Alma 35:14). The people of Ammon then left Jershon for the land of Melek to give place for the Nephite armies, and war began in the eighteenth year of the reign of the judges (Alma 35:13).
The Nephites gathered their armies in Jershon to meet the advancing Lamanites and Zoramites (Alma 43:4). Antionum, the Zoramite land, lay to its south (Alma 31:3). Rather than fight at the borders of Jershon, the Lamanites detoured through the wilderness toward Manti by the head of the river Sidon (Alma 43:22). Moroni left part of his army in Jershon to hold the city and marched the rest to Manti (Alma 43:25).