Correction: An earlier version of this commentary reported incorrectly that Air Force Gen. Curtis LeMay spoke to the New Yorker in 1995 about the scale of U.S. bombing during the Korean War. LeMay died in 1990. The quote came from a 1984 interview of LeMay by the Office of Air Force History; it was included in the magazine’s article on the general in 1995. The following version has been updated.

North Koreans pay their respects at the statues of former North Korean leaders Kim Il Sung (L) and Kim Jong Il on Lunar New Year in this February 19, 2015 photo. (Kcna/Reuters)

Blaine Harden, a former Post reporter, is the author of the book “The Great Leader and the Fighter Pilot.”

North Korea cheered this month when a man with a knife and a history of violent behavior slashed the face of Mark Lippert, the U.S. ambassador to South Korea. The attack in Seoul was “a knife shower of justice,” North Korea said, praising it as “deserved punishment for warmonger United States.”

If that sounds mean-spirited, consider this: For years, North Korea has taught schoolchildren to bayonet effigies of U.S. soldiers. Under its young dictator, Kim Jong Un, the government has suggested it was prepared to nuke Washington, Austin and Southern California. More than 40 years ago, Kim Il Sung, the “Great Leader” who founded the family dictatorship that rules North Korea, said there was “no secret” about his country’s behavior: “What is most important in our preparations [for war] is to educate all the people to hate U.S. imperialism.”

Where does the hate come from?

Much of it is cooked up daily in Pyongyang. Like all dictatorial regimes, the Kim family dynasty needs an endless existential struggle against a fearsome enemy. Such a threat rationalizes massive military spending and excuses decades of privation, while keeping dissenting mouths shut and political prisons open.

The hate, though, is not all manufactured. It is rooted in a fact-based narrative, one that North Korea obsessively remembers and the United States blithely forgets.

The story dates to the early 1950s, when the U.S. Air Force, in response to the North Korean invasion that started the Korean War, bombed and napalmed cities, towns and villages across the North. It was mostly easy pickings for the Air Force, whose B-29s faced little or no opposition on many missions.

The bombing was long, leisurely and merciless, even by the assessment of America’s own leaders. “Over a period of three years or so, we killed off — what — 20 percent of the population,” Air Force Gen. Curtis LeMay, head of the Strategic Air Command during the Korean War, told the Office of Air Force History in 1984. Dean Rusk, a supporter of the war and later secretary of state, said the United States bombed “everything that moved in North Korea, every brick standing on top of another.” After running low on urban targets, U.S. bombers destroyed hydroelectric and irrigation dams in the later stages of the war, flooding farmland and destroying crops.

Although the ferocity of the bombing was criticized as racist and unjustified elsewhere in the world, it was never a big story back home. U.S. press coverage of the air war focused, instead, on “MiG alley,” a narrow patch of North Korea near the Chinese border. There, in the world’s first jet-powered aerial war, American fighter pilots competed against each other to shoot down five or more Soviet-made fighters and become “aces.” War reporters rarely mentioned civilian casualties from U.S. carpet-bombing. It is perhaps the most forgotten part of a forgotten war.

The Kims, though, have kept memories of the war and the bombing terrifyingly fresh. North Korean state media dress up the historical record in a Big Lie, claiming that Americans and South Korea sneakily started the Korean War and that Kim Il Sung brilliantly won it against overwhelming odds. (The Chinese don’t get much credit for fighting the United States to a draw.) State media warn that, sooner or later, the Americans will strike again.

“It is still the 1950s in North Korea and the conflict with South Korea and the United States is still going on,” says Kathryn Weathersby, a scholar of the Korean War. “People in the North feel backed into a corner and threatened.”

There is real value in understanding this paranoid mind-set. It puts the calculated belligerence of the Kim family into context. It also undermines the notion that North Korea is merely a nut-case state.

Since World War II, the United States has engaged in an almost unbroken chain of major and minor wars in distant and poorly understood countries. Yet for a meddlesome superpower that claims the democratic high ground, it can sometimes be shockingly incurious and self-absorbed. In the case of the bombing of North Korea, its people never really became conscious of a major war crime committed in their name.

Paying attention in a democracy is a moral obligation. It is also a way to avoid repeating immoral mistakes.

And if North Korea ever does change, if the Kim family were overthrown or were to voluntarily loosen its chokehold on information, a U.S. apology for the bombing could help dispel 65 years of hate.