Quoting, of all places, the Washington Post:
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.