Adding a little bit more from the same chapter because it's interesting and tragic, albeit slightly off-topic

One may imagine the steel nerves required of the DPRK’s leaders, sitting in their deep bunker under Moran Hill in Pyongyang observing a lone B-29 simulating the attack lines that had resulted in the devastation of Hiroshima and Nagasaki just five years earlier, each time unsure of whether the bomb was real or a dummy. But then, they survived the kill-all, burn-all, loot-all campaigns launched under General Tōjō’s command in the 1930s. Successive American administrations have foolishly underestimated this leadership and have paid the cost for it in American lives. “War is a stern teacher,” Thucydides memorably wrote; indeed, it is the supreme teacher of one’s memory. As Nietzsche put the point in discussing human “mnemotechnics,” the oldest psychology on earth is that which must be “burned” in: “only that which never ceases to hurt stays in the memory.”

The declassified record shows that American commanders also considered the massive use of chemical weapons against Sino-North Korean forces. In penciled diary notes written on December 16, 1950, Ridgway referred cryptically to a subcommittee on “clandestine introduction [of] wea[pon]s of mass destruction and unconventional warfare.” I know nothing more about this item, but it may refer to Ridgway’s request to MacArthur that chemical weapons be used in Korea. The original of Ridgway’s telegram is unavailable; one author claims that this was merely a request for tear gas, but that would not explain MacArthur’s reply on January 7, 1951, which alluded to the laws of war, and read as follows: “I do not believe there is any chance of using chemicals on the enemy in case [American] evacuation is ordered. As you know, U.S. inhibitions on such use are complete and drastic.” The next day, in a conference with General Almond and others, the transcript says, “If we use gas we will lay ourselves open to retaliation. This question has been taken up with General MacArthur for decision. We have requested sufficient quantities to be shipped immediately in the event use of gas is approved.”

Without the use of “novel weapons”—although napalm was very new at the time—the air war nonetheless leveled North Korea and killed millions of civilians before the war ended. North Koreans will tell you that for three years they faced a daily threat of being burned with napalm; “you couldn’t escape it,” one told me in 1981. By 1952 just about everything in northern and central Korea was completely leveled. What was left of the population survived in caves. The North Koreans created an entire life underground, in complexes of dwellings, schools, hospitals, and factories. The Japanese built many tunnels and caves on the peninsula during World War II, for defense and storage of ammunition and equipment in anticipation of a ground campaign across the mainland against the home islands. During the Korean War, Korean and Chinese forces built massive underground installations because they had lost control of the air and because of well-grounded fears of nuclear attack. Commanding General Peng Dehuai’s memoirs estimated that 1,250 kilometers of tunnels and 6,000 kilometers of trenches were dug just along the war front itself, in and behind the current DMZ.