The japanese soldier is depicted as a cruel person that is responsible for the murder of american soldiers. The drawing shows him hitting a helpless american soldier; by showing an american soldier with his hands tied, the author of the poster wants to demonstrate the evil of these "japs". It's objective is to deshumanize japanese soldiers, by calling them "murdering jap", the poster states that their only purpose is to damage people.
As we can see, this poster is part of a propaganda campaign to depict the japanse as the enemy and inject courage to the people in the war.
