No war has redrawn the world's religious geography so quickly. The peace dissolved four empires at once. The Ottoman collapse ended the last widely recognised caliphate: the office survived the sultanate by only two years before the new Turkish republic abolished it in 1924, leaving Sunni Islam without the institution that had, at least in name, headed it for thirteen centuries, an absence that has shaped Islamic political thought ever since. The Ottoman provinces became British and French mandates, and the Balfour Declaration of 1917, folded into the Palestine mandate, set the course toward a Jewish national home, entangling the holy land's three faiths in a new political frame.
Russia's defeat and revolution delivered the world's largest Orthodox church into the hands of a state committed to atheism, opening seven decades of antireligious persecution. In defeated Germany, the collapse of the throne ended the old union of altar and crown, and out of the war's disillusion came the theology of crisis: Karl Barth's commentary on Romans, written in its shadow, broke with the liberal theology that had blessed the war and turned Protestant thought toward a God wholly other than the nations.
Among the bereaved millions the war drove a great surge of spiritualism, seances and memorial mysticism, championed publicly by figures such as Arthur Conan Doyle, while war memorials, two minutes of silence and poppies built a new civic liturgy of remembrance that the modern world still keeps each November.