The Ammonite Defenders were the Nephite armies set between the land of Jershon and the land of Nephi to protect the people of Ammon, the converted Lamanites also called the Anti-Nephi-Lehies, after they were given Jershon as an inheritance. By the voice of the people the Nephites granted them Jershon, on the east by the sea and south of Bountiful, and stationed armies along the border to guard them (Alma 27:22-24).
The people of Ammon would not take up arms against the Lamanites for fear of sinning, a fear that came from the repentance they felt over their former murders and wickedness. In exchange for the protection, they agreed to give the Nephites a portion of their substance to maintain the armies (Alma 27:23-24).