Should the U.S. have bombed the Auschwitz concentration camp during World War II?

That’s one of several “what if?” questions Ken Burns tackles in his latest documentary The U.S. and the Holocaust, which airs over three nights beginning Sept. 18 on PBS.

A running theme in the six-hour-plus feature, co-directed with Lynn Novick and Sarah Botstein, is what the U.S. could have done to avoid the worst of the atrocities that resulted in the deaths of approximately six million European Jews.

Deborah Lipstadt, the current Special Envoy to Monitor and Combat Antisemitism, argues that bombing Auschwitz would have sent “a message” to the Germans that “‘we know what you are doing. We cannot abide what you are doing. This is our response to what you are doing.’”

Leaders at the time considered attacking the concentration camp where at least 1 million people were put to death, but worried that it wouldn’t be effective. During the war, aerial bombing was imprecise; only one out of five bombs hit within five miles of its target. U.S assistant secretary of War John McCloy claimed President Franklin D. Roosevelt rejected the idea, worried the Germans would just rebuild the camp somewhere else.

“It is one of those tragic questions,” Rebecca Erbelding, Rescue Board: The Untold Story of America’s Efforts to Save the Jews of Europe, says in the film. U.S. authorities weighed the risks of doing nothing at all with the risk of bombing prisoners and people who would have survived. “No matter what we did, I think we’d look back and wonder what would have happened had we done the other thing.”

Ken Burns believes there’s no question that the U.S. could have done more. He believes that even if ten times as many refugees were let in, it wouldn’t be enough. “Despite the fact, as we say in the intro of the film, that the United States let in more people than any other sovereign nation—we didn’t do enough. We failed,” Burns tells TIME.