There is no enemy quite like one who was once a brother. Alma had been one of the priests of Noah, together with Amulon. Amulon now has the power of a Lamanite king behind him, and certainly harbored animosity toward Alma, likely for multiple reasons. That animosity led to persecution.
Mormon tells the tale of two peoples who were Nephites among the Lamanites; the people of Limhi and the people of Alma. Both were peoples who were under Lamanite control and both were forced to provide the Lamanites benefits from their labors. Although they were similar, Mormon highlights the differences.
Limhi’s people decided to attempt a military solution, but failed three times. Only after great tribulations did they remember their God and become sufficiently repentant that they could have God hear and respond to their prayers. Alma’s people, finding themselves in similar difficult circumstances, turn immediately to prayer.