The Nobel Prize recipient Alfred Hermann Fried a significant yet less celebrated pacifist at the turn of the twentieth century in both Austria and Germany, developed his "scientific pacifism" in his theoretical writings, Handbuch der Friedensbewegung (1905, 1911, 1913) and Die Grundlagen des revolutionären Pacifismus (1908). Less studied is his Kriegs-Tagebuch, which he kept during the war and published in four volumes in 1918–1920. This essay examines how the constitutent parts of "scientific pacifism" held up during the war, which Fried experienced from his forced emigration in Switzerland. It considers his views on internationalism, leadership, militarism, rearmament, economics, democracy, and the treaties ending the war. The essay argues that Fried remained steadfast in his passionate and rigorous convictions against war even if he came less sanguine, but not ultimately despondent, about the establishment of a stable peace.