In 1917, on the eve of America's entry into the First World War, Wood returned to the Army as an Infantry Lieutenant Colonel. He served in Europe with the 42nd (Rainbow) Division and was promoted to colonel. Later in the war Wood would serve as transportation director for the entire American Expeditionary Forces in France. Toward the end of the war, he was promoted to brigadier general and made acting Quartermaster General of the Army. In June 1916, prior to America's entry into the war, Wood's brother, Captain Stanley Wood, was killed in action while serving as a volunteer in the Canadian Army.