The United States entered World War I in 1917 as an associated power on the allied side of Britain and France. The government didn’t have time to waste while its citizens made up their minds about joining the fight. How could ordinary Americans be convinced to participate in the war “over there”?
Posters—which were so well designed and illustrated that people collected and displayed them in fine art galleries. They could be pasted on the sides of buildings, put in the windows of homes, tacked up in workplaces, and resized to appear above cable car windows and in magazines.
Source: Rare Historical Photos.com