Ken Burns shares what he learned making his latest documentary, "The Vietnam War"

Filmmaker Ken Burns said a "good deal" of the divisions in the United States today originated in the era that his latest documentary film series -- "The Vietnam War" -- depicts. 

Unpacking events surrounding the Vietnam War -- which included a White House obsessed with leaks and a crisis of the free press -- helps Americans better understand the current environment, Burns said, previewing the film on CBS News' "Face the Nation" Sunday. 

"The Vietnam War," an 18-hour film by Burns and Lynn Novick, will air on PBS in September. 

Burns and Novick spent 10 years on the film, which focuses on an era he called probably the most important in the second half of the 20th Century for Americans. In this project, like other projects, the film crew went in "confident" that they knew something about the history, then faced an almost "daily humiliation" realizing how little they actually knew, Burns said. 

Burns said the passage of decades, and increased scholarship on the topic, gives "new insight," Burns said. With the "passage of time, you have perspective," Burns said.