After the First World War the belief became substantially widespread among developed countries that the venerable institution of war should be abandoned from their affairs. It was an idea whose time had come. Historically, the war does not seem to have been all that unusual in its duration, destructiveness, grimness, political pointlessness, economic consequences or breadth. It does seem to have been unique in that (1) it was the first major war to be preceded by substantial, organized anti-war agitation, and (2) for Europeans, it followed an unprecedentedly peaceful century during which even war enthusiasts began, perhaps unknowingly, to appreciate the virtues of peace. Thus the war served as a necessary catalyst for opinion change. The process through which the change took place owes much to British war aims and to their efforts to get the United States into the war. The article concludes with some reflections on the historical movement of ideas.