During World War I, which the United States entered in April 1917, Reinhardt was promoted to temporary major. Recognized as an effective trainer and instructor, he did not see service overseas during the war and instead remained in the United States, where he served at Camp Fremont in California, Camp Pike in Arkansas, and Camp Hancock in Georgia, primarily as an instructor of infantry tactics and in the use of machine guns. In January 1919, just two months after the war came to an end with the signing of the Armistice with Germany, he was assigned as commander of the 41st Machine Gun Battalion at Camp Custer, Michigan. In 1919 and 1920 he also commanded the Camp Custer Convalescent Center.
In 1942, after the American entry into World War II, Reinhardt attended the Divisional Commanders Course at the U.S. Army Command and General Staff School at Fort Leavenworth, Kansas and was then promoted to the two-star rank of major general on April 17, 1942 and also appointed the first CG of 76th Infantry Division.[4]
From December 7, 1942 to December 1943, Reinhardt commanded XIII Corps.
Reinhardt returned to the United States in August 1945 and commanded the Infantry Replacement Training Center at Camp Robinson, Arkansas. He served in this capacity until September 30, 1946, when he retired from the military service.
^United States Military Academy. The Register of Graduates and Former Cadets of the United States Military Academy at West Point: 2004. Connecticut. Elm Press. 2004. p. 2:110
^Empric, Bruce E. (2024), Uncommon Allies: U.S. Army Recipients of Soviet Military Decorations in World War II, Teufelsberg Press, p. 58, ISBN979-8-3444-6807-5