Ken Burns told me that during the seven years he worked on his forthcoming three-part documentary, "The U.S. and the Holocaust," he struggled with the "opacity" of the unfathomable fact that 6 million Jews were killed by Germany during World War II.

• So the legendary documentarian said he opens with "a young, beautiful German woman leaning out a window and her parents or two other people come into the frame. And you're hearing that there were 9 million Jews in Europe in 1933. By 1945, two out of three are dead."

The film — directed and produced by Burns, Lynn Novick and Sarah Botstein — will air on PBS on Sept. 18, 19 and 20, from 8 to 10 p.m. ET.

• "The U.S. and the Holocaust" is 90% black and white — archival images and interviews. "The evidence," Burns calls it.

Why it matters: "I will not work on a more important film," Burns told me during our half-hour phone interview.

• "It might be possible to look back at 'The Civil War' and World War II and 'The Roosevelts' ... and say they might be equal. But I will not work on a more important film than this ... in terms of gravity of the subject."

I asked Burns, age 68, how he kept the film raw and real — without making it so painful that it's unwatchable.

• "We're in the job of calibration," he said. "I live in New Hampshire — we make maple syrup up there. We collect 40 gallons of sap for every gallon of maple syrup."
• On the Holocaust, he said, you "have to disarm the unexploded bombs of false scholarship, of misinformation on a conventional wisdom level, of the superficial information that many people — most people — seem to have."
• "You do not want to privilege the perpetrators. Much of the footage that's taken by German and their allies, you have to use very sparingly. ... You don't want people to be either turned away or, I hate to say this, even drawn to the horror in the wrong way. It has to be calibrated."

Asked about lessons for today, Burns said: "We're not unmindful that, as Mark Twain says: 'History doesn't repeat itself, but it rhymes.'"

• "As the film progressed through the last six or seven years," he said, "we began to realize just how terrifyingly rhyming these stories and moments and individuals and actions were with our present moment."

"The worst thing [would be] to wag your finger," he added. "You just have to understand that the things that became so intolerably out of control with the Nazi regime are not alien to any other culture."

• "The road down authoritarianism doesn't end well for people."

"We want to remind people of the frailty and the complexity — and, at times, the majesty of the human project, and that it's really important to be self-aware," Burns told me.

• "We're in an age where we're proscribing how much history we can teach. We're limiting what subjects we can teach. We're trying to shield people from an actual powerful and, at times, very disturbing truth about us."

"We have to be careful," Burns added. "It was hoped that the Constitution would be a machine that would go [on its own]. There is no guarantee of that. We are the guarantors. We are the mechanics of democracy."