Ammonites (Book of Mormon)
According to the Book of Mormon, the Ammonites (//) were a group of Lamanites who had been converted to the religion of the Nephites by the missionary efforts of Ammon and his brothers. They rejected the traditions of their fathers and embraced the traditions of the Nephites. To distinguish themselves from the Lamanites, they took upon themselves the name Anti-Nephi-Lehies. The name refers to the original leaders that lead their family out of Jerusalem and, guided by God, traveled to the Americas: Nephi and his father Lehi. In the usage here, "Anti" means "to imitate".
Prior to their conversion, the Ammonites were Lamanites, traditional enemies of the Nephites, and they had taken part in wars and battles against the Nephites. Following their conversion, they felt that their past sins were so great that they took a vow to never shed blood again and to avoid all forms of warfare, even in their own defense. To indicate the serious covenant they were making to this end, they buried all their weapons deep in the ground.
Forced from their homeland by their former Lamanite brethren, the Ammonites were re-located to the Nephite land of Jershon and the Nephites took up their defense in their behalf. The Ammonites, in return, provided provisions and material to help support the Nephite armies. When the Nephite and Lamanite nations entered into escalated warfare, the Anti-Nephi-Lehies volunteered to help the Nephites fight. Helaman, who was the leader of the church at that time, forbade them from doing so for fear that God would punish them for breaking their oath. Instead, they sent their sons who had not taken their oath to fight under the command of Helaman. This group was called the two thousand stripling warriors.
The history of the names of the Ammonites
The name that the converted Lamanites used to discern themselves from the Lamanites was in Anti-Nephi-Lehies. Following their relocation to the land of Jershon they were known by the Nephites as the people of Ammon.