Before the knives, pokers and axes come out, let me add I’m drawing any equivalences between the those different events. Indeed, it’s more about bringing out contrasts in context I think. Maybe I should have retracted that one…
Anyway, back to the original question. A reference I’d recommend is Downfall by Richard Frank. I think it looks at the end of Pacific War in such a way that nobody gets a whitewash and events are kept in perspective.
LeMay was brought in to replace Hansell. Hansell went by the USAAF “book” and sent the B-29s to conduct daylight high-level “precision” attacks on specific Japanese industrial facilities with high explosive bombs. He failed to make much of an impact partly because the weather and high altitude winds were worse than over Germany. The strain caused by long flights at high altitude also caused problems with the B-29’s engines. There was enormous political pressure to get results given the resources that had been poured into the difficult B-29 development and mass production program. The AAF chief Hap Arnold felt it as the B-29’s main champion and he passed every bit of on to the field commanders.
LeMay was a driven individual willing to throw the book aside. Japanese cities had long been noted to be more tightly-packed with flammable and lightly-constructed buildings than German cities. LeMay studied the aerial photos and saw that the anti-aircraft artillery had very few 20 to 40 mm automatic cannon of the sort that would make low-level attacks dangerous. At the same time, the Japanese lacked a strong night fighter force. So he sent the B-29s in at low level, at night, with mostly incendiaries to area bomb. This was much more like the way the British had conducted their bombing campaign against the Germans. The results were spectacular firestorms that ate the heart out of the residential districts of Japanese cities (the Navy was to complain that this didn’t seem to effect the rate of Japanese aircraft production as evidenced by the waves of Kamikazes coming after their ships.). Once LeMay got going along these lines, a terrible sort of momentum built up. The spectacular became routine. Indeed, he was eventually given a list of cities to NOT firebomb in order that atomic bombs could be used on them instead.
If you take away the long term effects of radiation, then I would say the atypical firestorm created over Tokyo on March 10, 1945 was more horrific than either the Hiroshima or Nagasaki atomic bombings. With Dresden and Tokyo the U.S. (along with the British) had already crossed the line into indiscriminate mass killing of civilians, and in light of those events, refusing to use to the atomic bomb because of moral considerations would be rather inconsistent: it was just a different technical means to the same end.
At the same time, I don’t think any American general or politician really set out knowingly to reach this state of affairs. The B-29 was envisioned as bombing Japanese factories into the ground. While damage to the surroundings was inevitable, it was to be minimized as much as possible. Without factories, without a Navy and without hope of victory the Japanese were supposed to sue for peace. But circumstances got in the way. Weather and mechanical difficulties ruled out that sort of bombing and the insistence on unconditional surrender (something Roosevelt committed America to rather without forethought back in Casablanca) hardened Japanese resolve.