“I didn’t know.”

Those were the first words my friend, a white guy named Dave, spoke to me the day after the first episode of “Roots,” which depicted the capture and enslavement of an African boy named Kunta Kinte. This was 43 years ago.

So you can perhaps imagine my frustration last year when the HBO miniseries “Watchmen” debuted with an episode depicting the Tulsa massacre of 1921 in which white mobs razed a thriving African American neighborhood, killing dozens of people. White readers reached out to me, shocked and amazed.

“I didn’t know,” they said.

I don’t blame them for not knowing any more than I did Dave. You cannot know what you have not been taught. But the not knowing is not accidental, either.

Which brings us to William Jackson Harper, a Black actor best known as the ethics nerd Chidi Anagonye on the NBC sitcom, “The Good Place,” and a disturbing experience he shared recently on social media. It seems he had been invited by a charity – Arts In The Armed Forces – to choose a film for cadets to watch and then moderate a Zoom discussion of its themes. From a list of films he provided, they picked “Malcolm X.”

Then they unpicked it. Harper says that, two days before the event, he was told two of the cadet academies would not participate for fear of violating Donald Trump’s recent so-called “Executive Order On Combating Race and Sex Stereotyping.” As Harper wrote, it “requires that federal and military institutions refrain from training material that promote a ‘pernicious and false belief that America is an irredeemably racist and sexist country; that some people, simply on account of their race or sex, are oppressors.'”

That may sound good, even noble, but it’s a load of fertilizer in a satin sack. Because the aim – and, as this incident indicates, the effect – of the order is really to discourage discussion of tough and touchy issues of race and gender. No “The Feminine Mystique.” No “Roots.” And no “Malcolm X.”

You have to marvel at the lengths to which this country will go to protect itself – to protect white people – from uncomfortable truths of race. In 2010, Arizona passed a law banning ethnic studies. In 2014, a school board member in Colorado proposed a policy that would wipe out Black history classes. Last year, a Bronx educator was accused of banning Black history. Now, there’s this.

And the reason is obvious. Jesus said, “The truth shall make you free,” but very often where race is concerned, the truth shall make you angry. Or disappointed. Or ashamed. So instead of truth, we are offered a version of history calibrated for the comfort of those white people unsettled by truth, one that does not ask them to look too deeply or question too much.

In order to force compliance, people in authority are increasingly willing to use policy and law. That’s the wall Harper ran up against. Which tells you how dangerous they consider this truth to be. To understand it is to understand America in a fundamentally different way, to see what you never saw before, hear what you never heard before, ask what you never asked before.

In effect, Trump imposed his own moral and intellectual cowardice upon those cadets. Those are traits too much in evidence these days where race is concerned, and they will serve us poorly in a nation growing more diverse by the hour. Yes, America’s racial truths are a threat to smug self-satisfaction, facile patriotism and easy myths. Some think that makes them too dangerous to know.

Actually, it makes them too dangerous not to.

Leonard Pitts Jr. is a columnist for the Miami Herald. Readers may write to him via email at: lpitts@miamiherald.com.