Bernie Sanders has yet to personally comment on perhaps the ugliest moment yet in his campaign for the presidency — a Nazi flag displayed Thursday at his rally in Phoenix.

But given what Sanders has learned in recent years about his family’s Holocaust history, the incident was likely deeply painful.

Sanders grew up in Brooklyn, the son of Jewish immigrants. His father Elias immigrated there from Poland in 1921 at age 17 to “escape the poverty and widespread anti-Semitism of his home country,” as Sanders recounted last year in an essay for the magazine Jewish Currents.

Like the offspring of other immigrants who left Europe before the rise of Adolf Hitler, Sanders’ father wasn’t especially loquacious when it came to details about his struggles back home.

Sanders knew his father, a paint salesman, had grown up hungry and the target of anti-Semitism during the First World War. He knew his father’s family — those who stayed behind — didn’t fare well years later during Hitler’s march across Europe.

That was about it.

But a few years ago, the remarkable details emerged when Sanders appeared on the PBS show “Finding Your Roots,” in which scholar Henry Louis Gates Jr. and a team of researchers retrace the ancestral lives of actors, politicians and other stars. (The Sanders episode also featured Larry David, his comedic doppelganger. It turns out they are distant cousins, which understandably blew both of their minds.)

Gates and his team discovered that Elias Sanders travelled to the United States by himself, with just $25 dollars. What he escaped was horrific. The town of Dobra was assailed not just by anti-Jewish forces but by townspeople who also wanted Jews out and took part in their plundering.

“Oh god,” Sanders says, as he reviews archival accounts. “This is the first I hear about the specifics. But you think about how vulnerable ... who’s there to protect you? There’s no law and order. There’s no police that you can go to because the police may be turning on you.”

“What I’m seeing,” Sanders says, “is totally horrific.”

Gates revealed more details. Synagogues were burned. Residents endured attacks from armed forces and armed peasants.

“My father grew up in a community where you did not know who you can trust,” Sanders says, putting his father’s childhood together for the first time. “Maybe the person you bought something from in two days would be ransacking your house. I mean, how do you live in that kind of environment? I mean, I knew it was tough. But now add all that together. That is a hell of a place to grow up in.”

Then Gates revealed what Elias left behind in the years that followed. He showed Sanders a picture of his extended family that he had never seen before. In it was a man about whom he knew almost nothing: Elias’ half-brother Abraham (though he often was known as Romek.)

Abraham was born with a withered right-arm. By 1939, he and the other family members were confined in a ghetto. Abraham was a high-ranking member of the ghetto council that kept order and served as an intermediary with German forces.

Gates found a letter Abraham had written to an aid organization describing life in the ghetto, and he asked Sanders to read it.

“The Jewish community,” Sanders read, “will not receive any external support. They will be condemned to inevitable death from starvation. Therefore, we beg your help.”

Gates says, “What do you think it must have been like for your father’s siblings to endure that? Can you imagine that?”

Sanders replies: “There comes a point where you really can’t imagine, you really can’t. How do we know what kind of horrors and pain people were feeling? I mean, it’s impossible, I think, for any person to know how people in that moment were feeling. But it was horrible. Unutterably horrible.”

Sanders did not know his uncle was on the ghetto council. He also didn’t know the extreme choice he faced one May day in 1942, when Nazi officers demanded he turn over a group of resisters so they could be executed.

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Abraham refused.

He was shot in the back of the head.

Gates showed Sanders a picture of the officer who ordered his uncle’s death.

“You look at people who look normal and you just wonder how people could descend to that type of barbarity,” Sanders says. “These are normal people. The have wives, they have children. How can you go around starving people or shooting people? Because you think that they are inferior? Or different than you are?”

Gates asks Sanders how he felt learning that a family member had stood up to the Nazis.

“I’m proud of his courage and willingly going to his own death in order to protect innocent people,” Sanders says. “I’m very, very proud that I have a family member who showed that type of courage and decency.”

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