Antionum was the land settled by the Zoramites, east of the land of Zarahemla, near the seashore, south of the land of Jershon, and bordering the wilderness to the south that was full of Lamanites (Alma 31:3). The Zoramites had fallen away from the Nephites, and Alma² and Amulek went there to preach to them on faith and on Christ. Those who accepted their words were cast out of the land and came over into Jershon, where the people of Ammon received them (Alma 35:6, 10-11).
The expulsion of these believers turned the Zoramites to anger against the people of Ammon, and they joined with the Lamanites in preparing for war against them and against the Nephites (Alma 35:10-11). A Lamanite army under Zerahemnah then gathered in Antionum (Alma 43:5). The Nephites prepared to meet them in Jershon, and when the Lamanites saw that Moroni had equipped his men with breastplates, shields, and thick clothing while their own force was largely unarmored, they would not attack and instead departed out of Antionum into the wilderness, heading by the head of the river Sidon toward the land of Manti (Alma 43:15, 18-22).