American propaganda posters during the Vietnam War often emphasized the importance of patriotism and duty, and sought to promote the war effort and win the hearts and minds of the American people.
One common theme was the portrayal of the Viet Cong and North Vietnamese soldiers as brutal and inhuman, though you won't find these on this site.
Another common theme was the importance of supporting the troops and the war effort. Many posters urged citizens to buy war bonds, conserve resources, and support the soldiers in the field. Some posters used images of heroic soldiers to encourage enlistment, while others emphasized the need for unity and sacrifice.
On the flip side, anti-war activists produced their share of propaganda posters protesting the conflict, often using psychedelic art and bright colors (popular with the counterculture movement). These posters often featured slogans such as "Make Love, Not War" and "Hell No, We Won't Go" and were used to protest the conflict.