On April 6, 1917, the United States declared war against Germany and entered the First World War, thus joining a conflict that had already been raging in Europe for more than two and a half years. U.S. government officials and private American relief agencies now faced a crucial public relations task: mobilizing popular enthusiasm for the war effort. One of the principal tools they used to achieve this goal was the propaganda poster. Large, colorful, and eye-catching posters blanketed First World War–era America. Appealing to American ideas about nationalism, patriotism, humanitarianism, and gender, posters urged U.S. citizens to commit fully to the war effort. They called on Americans to support U.S. and Allied troops while teaching them to abhor the enemy. They encouraged the American public to volunteer their time and donate their hard-earned money to wartime causes. As primary sources, these posters offer many insights into First World War–era American culture. It is important to remember, however, that those who viewed posters did not necessarily accept or understand the messages that their producers intended to convey. Posters are thus valuable primary sources yet must not be treated as direct reflections of the past.