Mosiah 23
Even Alma knew that if the people could always have a just king it would be well despite the fact that he was probably young during any of the reign of Zeniff who was the only good king he might have known prior to this (later in his life, of course, he will meet Mosiah). The people of Alma had years in the land of Helam before the Lamanite army stumbled upon them. They left before Limhi became king and Limhi had two years of peace plus 3 wars against the Lamanites plus bondage before they became free – which preceeded the discovery of the land of Helam by the Lamanites.
I’ll bet that the king of the Lamanites made Amulon the ruler thinking that the people of Alma would be more receptive to having a Nephite ruling over them for him instead of a Lamanite.