Alma 59:6

yea, even those which had been compelled to flee from the land of Manti, and from the land round about, had come over and joined the Lamanites in this part of the land;

Some Nephites had been forced to run away from Manti and the areas around it. They had joined the Lamanites.

Nephite refugees who’d been forced to flee from Manti and the surrounding areas had crossed over and joined up with the Lamanites in that region.