Enlisting in the U.S. Army in 1910, Middleton was first assigned to the 29th Infantry Regiment, where he worked as a clerk. Here he did not become an infantryman as he had hoped, but he was pressed into service playing football, a sport strongly endorsed by the army. Following two years of enlisted service, Middleton was transferred to Fort Leavenworth, Kansas, where he was given the opportunity to compete for an officer's commission. Of the 300 individuals who were vying for a commission, 56 were selected, and four of them, including Middleton, would become general officers. As a new second lieutenant, Middleton was assigned to the 7th Infantry Regiment in Galveston, Texas, which was soon pressed into service, responding to events created by the Mexican Revolution. Middleton spent seven months doing occupation duty in the Mexican port city of Veracruz, and later was assigned to Douglas, Arizona, where his unit skirmished with some of Pancho Villa's fighters.