Fleming attended the School of Submarine Defense at Fort Totten in 1905, and be subsequently assumed command of the 15th Field Artillery Battery.

[4] He participated in combat during the Meuse–Argonne offensive, and he received the Distinguished Service Medal for his efforts there,[3][5] as well as two oak leaf clusters and the Legion of Honour.

General Fleming commanded with distinction the 158th Field Artillery Brigade, 83d Division, displaying aggressive leadership and the highest professional attainments.

He contributed materially to the successful operations of the Infantry units to which his brigade was attached during the Meuse-Argonne offensive by the timely and accurate artillery support furnished by his regiments.

In command of his own brigade again, Fleming supported the 17th French Army at the Battle of Verdun, and after rejoining the 32nd Infantry, they took part in the occupation of Germany until July 1919.