During world war 1 many people didn't want to join because they would most likely die. Posters were one of the ways to communicative to them to join the fight. It made it appear great and honorable even though it was deadly. "During World War I, the impact of the poster as a means of communication was greater than at any other time during history"
https://www.loc.gov/collections/world-war-i-posters/about-this-collection/
https://www.loc.gov/collections/world-war-i-posters/about-this-collection/