The First World War was fought by the most churchgoing generation of the modern age, and, as Philip Jenkins has shown in detail, every belligerent nation framed it in openly religious terms. German soldiers marched under belt buckles reading Gott mit uns while court preachers proclaimed a holy German mission; British and French pulpits answered with the language of crusade and martyrdom; Russia went to war under the banners of Orthodox holy Russia. The dead of the Battle of Mons acquired guardian angels in a legend that swept Britain in 1915, apparitions and prophecies multiplied in every combatant country, and the industrial slaughter of the Western Front was read through the Book of Revelation by clergy and soldiers alike.
The war was formally a holy war on one front. In November 1914 the Ottoman sultan caliph, at Germany's urging, proclaimed jihad against the Entente powers, an attempt to raise the Muslim subjects of Britain, France and Russia that largely failed but that entangled the institution of the caliphate in the war's outcome. Within the Ottoman Empire the war years brought catastrophe to the Armenian Christians, whose deportation and mass killing from 1915 remains the war's darkest religious legacy.
Chaplains of every faith served in the trenches, Sikh, Hindu and Muslim soldiers of the Indian Army fought across Europe and the Middle East, and the war carried the religions of half the world into contact, and into graves, on a scale never seen before.