First World War represents a key moment in the History of Propaganda. Never before in another conflict propaganda had spread with such intensity. United States, which decide to enter the conflict in April 1917, developes a massive propaganda labor through the CPI or Creel Committee to ensure that public opinion, in favour of staying out of the war, support it. Among the many propaganda vehicles directed to the rear employed by this Committee, we will focus on the poster as its impact as a propaganda medium in this conflict was higher than in any other period of history. The poster directed to the rear faithfully followed two of the fundamental principles of political propaganda: simplification and distortion. Many of them have a ruthless enemy with evil values (Germany) against a hero with noble values (USA) who is forced to stop him. Making use of the stereotype and emotions, basic tools for any successful propaganda campaign according to Edward Bernays, they convince the public of the need to participate in the conflict.