World War I is the first conflict in which the implied States create a few institutional structures for the propaganda activity. An activity that had to provide them with the harmony between the plans that the States had for war and what the citizens thought a priori. There is analysed, as a case study, the activity of the U.S. Publicy Bureau that was able to transform a pacific and isolationist citizenship into a belligerent and frightened society before the German enemy.; La hipótesis de la espiral del cinismo (Capella y Jamieson, 1997) relaciona la cobertura que realizan los medios sobre los asuntos públicos con el descenso del interés ciudadano por los temas políticos, con la menor participación en política y con la desconfianza hacia l...