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.