On January 2, 1900, Butt was commissioned as a captain in the United States Volunteers (an all-volunteer group which was not part of the regular United States Army but was under the regular Army's control). He had long admired the military, and no one in his immediate family was serving in the armed forces at the time the Spanish–American War broke out. Although Butt's literary career was taking off, his family's long involvement with the military and his desire to represent his family in the army led him to enlist. Adjutant General of the U.S. Army Henry Clarke Corbin was influential in encouraging him to enlist.