Why did the u. s. government propaganda try to portray the japanese as sinister?

During the Second World War, especially in the early years, American propaganda tried to portray the Japanese as being sinister in order to inspire Americans willingness to engage in warfare against Japan. Before the attack on Pearl Harbor, most American citizens did not support going to war against the Axis powers.  American propaganda, in turn, focused on portraying the Japanese as being sinister in order to frighten Americans and turn them against the Japanese.  

Because the discoveries were some of the oldest human remains.