Cliff notes version: The Japanese had correctly deduced that the Allies were going to land on the Japanese home island of Kyushu, and correctly deduced the 3 landing beaches. The Japanese planned a furious defense called "Ketsu-Go" ("Decisive Operation") on Kyushu, with the goal of making the invasion so costly that the Americans would negotiate terms favorable to the Japanese. The American planners had anticipated 350,000 defenders on Kyushu, while the Japanese had actually managed to deploy 750,000 defenders there. US Navy CNO Adm. King was about to withdraw his support for the invasion when these new figures came to light.
The Japanese Army was in de facto control of the government as the war drew to its close. Even after the 2nd atomic bomb was dropped on Nagasaki, the Army refused to accept the terms of the Potsdam Declaration, and insisted on no Allied occupation of Japanese territory, Japanese self-disarmament, and Japanese control of War Crimes trials none of which would have been acceptable to the Allies. Had the Emperor not personally intervened to end the war, Japanese Army intransigence would have led to a pre-invasion continuation of atomic bombings, a destruction of the Japanese rail network (leading to mass starvation as the Japanese merchant fleet had already suffered such losses that the Japanese had trouble moving food between the home islands) and selected firebombing of Japanese cities.
Frank's conclusions are the atomic bombs negated the Ketsu-Go strategy in the eyes of the Emperor and more moderate Japanese leaders, and were the immediate cause of the Emperor's decision to terminate the war. The Japanese knew that the army in Manchuria was a lost cause if the Soviets launched a major attack against it (one Japanese message described the Manchurian Army as “hopeless”). Frank also concludes that 150,000 Asians were dying each month under the brutal Japanese occupation, and that these lives are never considered in the calculation of the human cost of the atomic bombings versus their effect on ending the war.
The bottom line is that Fat man and Little Boy saved millions of lives, most of those lives being Japanese. A fanatical, firebombed, invaded and starving Japan in which poorly-equipped Japanese civilians were trained to resist Allied invading forces would have rapidly doubled the eventual cost of over 3 million dead Japan lost during the war.
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Wholeheartedly agree. It may be the best WWII book that I’ve ever read. If you like the topic I recommend one that I about halfway through now - Pacific Crucible by Ian Toll. It deals with the first two years of the war in the Pacific.