Montague was against the First World War prior to its commencement, but once it started he believed that it was right to support it in the hope of a swift resolution. In 1914, Montague was 47, which was well over the age for enlistment. But in order to enlist, he dyed his white hair black to enable him to fool the Army into accepting him. H. W. Nevinson would later write that "Montague is the only man I know whose white hair in a single night turned dark through courage." He began as a grenadier-sergeant, and rose to lieutenant and then captain of intelligence in 1915. Later in the war, he became an armed escort for VIPs visiting the battlefield. He escorted such personalities as H.G. Wells and Bernard Shaw. After the end of World War I he wrote in a strong anti-war vein. He wrote that "War hath no fury like a non-combatant." Disenchantment (1922) was one of the first prose works to strongly criticise the way the war was fought, and is a pivotal text in the development of literature about the First World War.
He returned to the Guardian, but felt that his role was diminishing as the years passed. He finally retired in 1925, and settled down to become a full-time writer in the last years of his life. He died in 1928 at the age of 61.