The Ruhr invasion of 1923 prolonged the antagonism of the First World War between France and Germany. Challenging this rhetoric of enmity from 1921, French veteran and Catholic politician Marc Sangnier angered many by inviting the 'enemy' to Paris. Through his audacious Peace Congresses, Sangnier placed himself at the centre of a broader European civic campaign for moral disarmament or the 'disarmament of hatred'. European détente after 1924 lent currency to such staged reconciliation and crossing of borders. Mining a variety of sources, both known and new, Gearóid Barry documents the Peace Congresses' surprising resonance and political ecumenism (embracing Quakers, secularists, socialists and the pope) while reconfiguring the transnational histories of youth movements, women's peace activism and Christian Democracy. Pledged to reject 'war culture', these peace activists shared excruciating new choices between peace and appeasement in the 1930s. This story casts new light on key questions in European history in the era of two World Wars.