How, and by what means, is peace constituted? In the first two decades of the twentieth century, a leading pacifist, Alfred H. Fried, set this fundamental question at the heart of the pacifist programme. Causal pacifism was the key term. The doctrine of causal or cause/effect pacifism is therefore rooted in an attempt to think systematically about the prerequisites and conditions for peace. Irrespective of whether or not this specific term was used by individual authors, causal pacifism was a key academic and practical issue in the classical pacifism debate. It is one of the great tragedies of the twentieth century that this concept declined in popularity among pacifist movements and finally became a non-issue. In a twentieth century marked by violence, war, genocide and mutual threats of destruction within the framework of deterrence, antimilitarism – for quite understandable reasons – came to dominate the pacifist agenda and shape its thinking and action. In short, causal pacifism and comparable approaches could therefore also be described as ‘constructive pacifism’ – a pacifism that is geared to the construction and architecture of peace.