Propaganda is a type of persuading strategy that aims at promoting a specific political position, usually by presenting bias information or using manipulation. Moreover, propaganda is also used in wars as a way to make people support or reject those nations participating in the conflict.
The poster presented is an example of propaganda because the purpose of this poster was to persuade U.S citizens to join World War I. This shows the poster promotes a political position as it wants to make U.S. citizens support the U.S. in the war. Moreover, this uses manipulation because Uncle Sam (a symbol of the U.S.) seems to accuse those who have not joined the army.
