Manning quickly reached the rank of first lieutenant and served on the Mexican Border during Pancho Villa Expedition, when his unit was called into Federal service in summer of 1916. Upon the United States' entry into World War I in April 1917, Manning's unit was redesignated 119th Infantry Regiment in September 1917 and he was promoted to captain. He served as regimental adjutant under Colonel John Van Bokkelen Metts at Camp Sevier, South Carolina and embarked with his regiment for France as the part of 30th Infantry Division in May 1918.
