I was watching a documentary called world War 2 in color on metflix. In episode 13, it tells how US strategic bombing with high explosive bombs was unsuccessful because the bombs missed. This was because the bombers had to fly high since Japan was out of range of fighter escorts. Just after this, it says that US strategic firebombing with fire bombs from low altitude was devastating, albeit with high US bomber casualties. Since this shows that they could stomach the loss, why not just use high explosives, or use incindieary at high altitude (since fire spreads)?
The first attacks on Japan were carried out from China. Bombing was conducted at high altitude in part because of a lack of fighter escort but it was inherent doctrine: in fact at the start of the war the USAAF bomber leaders were divided on the matter, with precision bombing from high altitude believed to be potentially very successful (based on tests in fairly ideal conditions). Brigadier General Haywood Hansell, who led the first raids from Pacific bases, was a committed believer in high-altitude bombing.
The B-29 was designed specifically to fly very fast and very high (for a heavy bomber), and at first it was employed in this way.
The problem was nobody understood the extremely fast winds blowing in the upper atmosphere over Japan, and attempts to hit particular factories, power stations and so on proved unsuccessful.
In January 1945 Hansell was replaced by Curtis LeMay. Bombing became more successful but losses increased as the air defences gained experience. It was difficult for the Japanese fighters to operate at the altitudes in question and they lacked firepower for such large targets, but their dedication to ramming, the vivid impression of crashing B-29s, the horrors of Japanese captivity and the small odds of rescue in a ditching situation combined to affect morale somewhat.
In late February LeMay, always a technician, statistician and analyst of industrial warfare, switched to a fire bombing strategy after tests and a successful raid in Tokyo. He decided that incendiary bombs were the key to success against Japan's wooden cities (bombing technique along such lines had long been under analysis in Europe, including by LeMay).
LeMay was a man to put effectiveness before doctrine, and he decreed that the B-29s should be loaded with a maximum weight of incendiary bombs, and to this end he stripped the B-29s or defensive armament, and ordered them to fly low and by night.
To start fires, a concentration of fire bombs is needed, which is why bombing from low altitude was so effective: the jet stream could not dissipate the bombs. In a bombing sense, using fire bombs to maximum effect is actually very difficult. Royal Air Force raids on Hamburg and Dresden were notoriously effective, creating 'firestorms' of self-feeding winds and combustible materials, but raids carried out with similar equipment in dozens of other places produced only light damage.