During World War I, the German threat was depicted as a mad, marauding gorilla with a bloody club.
Recently, an American political cartoon asked the question, "What Would Mohammed Drive?" and answered it with a drawing of a man dressed in Muslim garb driving a rental truck with a missile hanging out the back.
"For most human beings, it takes an awful lot to allow them to kill another human being," said Anthony Pratkanis, a psychology professor at the University of California, Santa Cruz. "The only way to do it is to justify the killing, to make the enemy look as evil as possible."
Propaganda, both by governments and the private media, has evolved over the years as media has evolved. But, some say, the principle remains the same.
"The secret in propaganda is that when you demonize, you dehumanize," said James Forsher, a film historian and documentary filmmaker who has studied propaganda films, and who is an assistant professor of mass communications at California State University, Hayward.
"When you dehumanize, it allows you to kill your enemy and no longer feel guilty about it," he said. "That is why during World War II, a lot of caricatures became animals. … You can kill a monkey a lot more easily than you can kill a neighbor."
Tales of atrocities also can dehumanize, as readers of William Randolph Hearst's newspapers learned when they got whipped up for the Spanish-American war with fake stories and sketches of Spanish atrocities that probably never happened. Arguably, it set a pattern for phony or embellished American wartime propaganda that would last at least through the Gulf War.
The Spanish-American War also delivered, perhaps for the first time, films gloriously recreating war events, Forsher said, and, "The government looked at it and went, 'Wow.'"
For World War I, the United States government created a propaganda office, and made sure to deploy movies in its propaganda arsenal. A cooperative Hollywood responded with films such as The Kaiser, the Beast of Berlin, To Hell With the Kaiser! and D.W. Griffith's Hearts of the World, which showed Germans throwing babies out windows, Forsher said.
On the other hand, an unfortunate group of filmmakers in 1917 put out The Spirit of '76, an epic about the American Revolution.
"Bad timing, as we were joining the war on the side of the British," Forsher said. "The Americans confiscated every print and threw the producer in jail."
Much of World War I's demonization propaganda seems blunt to modern eyes. A U.S. Army recruitment poster depicted a crazed gorilla wearing a German spiked war helmet, holding a bloody club in one hand and a half-ravished woman in the other, and stepping onto the shores of America. It said, "Destroy this Mad Brute."
In fact, the anti-German propaganda began even before America entered the war.
"In 1914, when war broke out, within weeks the image of Germans and Germany became filled with every symbol of barbarism that people could think of," said Jay Winter, a Yale University history professor whose specialty is World War I. "It had to be something where the war was the sons of light against the sons of darkness. And the reason was to muzzle dissent."