The regiment was drafted into Federal Service for World War I, 5 August 1917 as part of the 40th Infantry Division. Reorganization after the 1st World War assigned the 158th Infantry to the 45th Infantry Division. On 16 September 1940, the declaration of the National Emergency, the 158th Infantry joined its parent organization, the 45th Division at Fort Sill, Oklahoma. After being relieved by divisions in campaign after campaign across the Pacific, the 158th Infantry was selected to spearhead the final invasion of Japan.