‘Whenever the actual historical situation sharpens the issue, the debate whether the Christian Church is, or ought to be, pacifist, is carried on with fresh vigour both inside and outside the Christian community.’ Thus wrote Reinhold Niebuhr, the American commentator on Christian ethics, in 1940, having himself been converted first to pacifism and then back again in the course of the interwar period. Final agreement in this debate is, of course, improbable. But this paper will argue that the Christian cases for pacifism and non-pacifism alike were clarified, at least in Britain and for several decades, by the extraordinary ‘sharpening’ of the issue afforded by the ‘actual historical situation’ in the era of the two world wars. The shock of the first world war produced unprecedented support for Christian pacifism; and the aggressions of the 1930s, culminating in the crisis of 1940–1 when Britain faced the possibility of invasion and defeat, provided a series of tests which only the most rigorously thought-out version of that faith could survive—but, having survived them, it could survive anything.