Mosiah 23: 30-39
Looking at the events that led the Lamanites to the land of Helam, I think it is very obvious that the Lord ordained this trial for Alma and his people. The Lamanites arrived as they sought the people of Limhi, but not before they had found the people of Amulon. I imagine that the Lamanite army felt fortunate that they found two more groups of Nephites to replace the group of captive Nephites that they had lost. They would likely fare better in the opinion of their king than if they had merely lost the one group of captives they had been charged with guarding.