Though patriotic at the outbreak of the war, he soon grew disillusioned. In 1915, he began publishing a magazine with a group of friends, featuring expressionist poetry and essays on art. He became one of its leading authors even while at the front, writing a number of short stories and two novels. The events of 1918 radicalised Mierendorff. He later recalled that, while serving on the western front in October, his unit closely followed news from Russia: "The common refrain of our evenings was that 'we want to be good Bolsheviks.'" In the magazine's final issue in November, he called for readers to take the initiative in the unfolding revolution.