On July 16, four companies gathered to hear the last instructions from Brigham Young and others in the Quorum of the Twelve Apostles of the Church. On the 17th, more volunteers were called to fill the Fifth Company of the Battalion. On the 18th, Brigham Young met with the officers and instructed them to be as fathers to their men and to remind them to pray always. He promised the men that if they were faithful in their duties, not one of them would fall in battle with the enemy, and that their only battles would be with wild beasts.
On July 21, 1846, the Mormon Battalion began the longest infantry march in history to the tune, "The Girl I Left Behind Me."