AP Photo Marines unloading and moving through a tree and branch strewn landing zone in South Vietnam on December 17, 1969. roadtodisaster_cover.jpg E quipped with fresh insights from the fields of cognitive science and psychology, Brian VanDeMark’s Road to Disaster: A New History of America’s Descent Into Vietnam examines how Lyndon Johnson and his Vietnam advisors, “best and the brightest,” as David Halberstam’s famously called them in his seminal work, unspooled the decisions that cost the lives of more than 58,000 Americans. VanDeMark, an associate professor of history at the U.S. Naval Academy, has taught courses on the Vietnam War for nearly 30 years. As a young historian, he assisted Robert McNamara, Lyndon Johnson’s Secretary of Defense, with his controversial 1995 memoir on the war and got to know other senior advisors like Clark Clifford, McNamara’s successor. After Vietnam, Americans embraced a less jaundiced view of veterans and military service, but VanDeMark also tells...