A U.S. Marine shows a message written on the back of his flack vest at the Khe Sanh combat base in Vietnam on Feb. 21, 1968 during the Vietnam War. The quote reads, "Caution: Being a Marine in Khe Sanh may be hazardous to your health." Khe Sanh had been subject to increased rocket and artillery attacks from the North Vietnamese troops in the area. (AP Photo/Rick Merron)