Alma 46:34

Now Moroni being a man which was appointed by the Chief Judges and the voice of the people, therefore he had power according to his will, with the armies of the Nephites, to establish and to exercise authority over them.

Moroni was a man chosen by the chief judges and the people. So he had power over the armies of the Nephites.

Since Moroni had been appointed by the chief judges and chosen by the people, he had full authority over the armies.