Alma 48

As war seemed to be eminent between the Nephites and Lamanites, Mormon comments that -

"...the Nephites were taught to defend themselves against their enemies, even to the shedding of blood if it were necessary; yea, and they were also taught never to give an offense, yea, and never to raise the sword except it were against an enemy, except it were to preserve their lives.
15 And this was their faith, that by so doing God would prosper them in the land, or in other words, if they were faithful in keeping the commandments of God that he would prosper them in the land; yea, warn them to flee, or to prepare for war, according to their danger..."   (Book of Mormon | Alma 48:14 - 15)

These last words would suggest the need to follow the prophet.  And Mormon then added -

"And also, that God would make it known unto them whither they should go to defend themselves against their enemies, and by so doing, the Lord would deliver them; and this was the faith of Moroni, and his heart did glory in it; not in the shedding of blood but in doing good, in preserving his people, yea, in keeping the commandments of God, yea, and resisting iniquity."  (Book of Mormon | Alma 48:16)

Again, the need for a prophet to make known where to defend themselves.