“I think that when Americans talk about the Vietnam War … we tend to talk only about ourselves. But if we really want to understand it … or try to answer the fundamental question, ‘What happened?’ You’ve got to triangulate,” says filmmaker Ken Burns of his celebrated PBS documentary series “The Vietnam War.” “You’ve got to know what’s going on. And we have many battles in which you’ve got South Vietnamese soldiers and American advisors or … their counterparts and Vietcong or North Vietnamese. You have to get in there and understand what they’re thinking.” Burns and his co-director Lynn...