What, then, provoked one U.S. Marine to let himself be videotaped apparently flinging a yelping puppy over a cliff, bursting into laughter at the sound of the animal’s body hitting the ground below? The tape of the apparent incident has rocketed around the Internet, provoking a firestorm of criticism. The motivation for such an act, if it did indeed occur, may be as complex and deep as the U.S. war that has dragged on for more than four years, experts told ABCNEWS.com. Chief among them: Having to live with the constant fear of being injured or killed might have led this Marine to take his aggression out on a defenseless animal, several psychologists said.