After the United States entered World War I in April 1917, Warren volunteered for an officer training camp, but was rejected due to hemorrhoids. Still hoping to become an officer, Warren underwent a procedure to remove the hemorrhoids, but by the time he fully recovered from the operation the officer training camp had closed. Warren enlisted in the United States Army as a private in August 1917, and was assigned to Company I of the 91st Division's 363rd Infantry Regiment at Camp Lewis, Washington. He was made acting first sergeant of the company before being sent to a three-month officer training course. After he returned to the company in May 1918 as a second lieutenant, the regiment was sent to Camp Lee, Virginia, to train draftees. Warren spent the rest of the war there and was discharged less than a month after Armistice Day, following a promotion to first lieutenant. Warren would remain part of the United States Army Reserve until 1934, rising to the rank of captain.