General LeMay expressed surprised at using the atomic bomb against the Japanese as he felt no military targets remained – his incendiary bombing had already destroyed nearly everything. The use of it is explained by Truman’s desire “that the [atomic] bomb would provide diplomatic benefits by making the Soviets more tractable” in postwar negotiations.
Incendiary Raids on Japanese Cities
“Army Air Force General Curtis LeMay ordered the American strategic bombers to begin night incendiary raids on Japanese cities rather than attempting daylight precision bombing of industrial centers. The [B-29] Superfortresses honed their skills in three raids against Tokyo. A single bombing raid in February  destroyed at least 25,000 buildings. But the most devastation attack of the war, more deadly perhaps than both atomic bombs together, occurred on March 9, 1945.
Two hundred seventy-nine Superfortresses, sortieing from Guam, Saipan and Tinian, dropped firebombs from 7,000 feet on Tokyo’s residential areas. The paper and wood city erupted in inescapable flames. Sixteen square miles were completely destroyed.
During this single raid, approximately 100,000 Japanese civilians were killed. Many died from being scalded as they tried to save themselves by crowding into the city’s canals, which boiled. By comparison, 100,000 civilians died in the Hiroshima nuclear attack, while 35,000 died at Nagasaki. The raid lasted three hours. American Superfortress pilots and crews in the last wave vomited in their aircraft from the stench of burning flesh carried to their mile-high altitude.
Over successive days, the B-29s progressed to other Japanese cities. The United States burned Nagoya, Osaka, Yokohama, then Kobe. Then the bombers moved on to the lesser cities. As Japan had few [anti-aircraft] guns left, the B-29s could strike with impunity.
Citizens were warned with leaflets that their neighborhood would be razed. But few had anywhere to go. By the end of the war, nearly 400,000 Japanese civilians would be killed, mostly in American bombing attacks.
[From] April 1 until mid-June 1945, less than three divisions of Japanese held out in Okinawa, without support….Fourteen Japanese divisions and five….brigades protected Kyushu. President Truman later claimed, and many American sources agreed, that an invasion of Japan could have cost a million American casualties. This is far from accurate.
General Dwight Eisenhower said that he felt that the atomic bombing was unnecessary from the point of view of saving American lives. Admiral William Leahy, Secretary of War Henry Stimson, Secretary of the Navy James Forrestal, and Army Air General Henry “Hap” Arnold, all thought that dropping the bomb was unnecessary. Japan was broken and would have surrendered without an invasion.
The Japanese had no fuel, fewer than 10,000 trucks, almost no ability to manufacture weapons or ammunition, nor to transport supplies to Japan. They had almost no tanks left and remained wholly unable to defend themselves from air attack. Famine and disease threatened most of the population. Millions of Japanese civilians remained homeless. One by one, their cities were being razed. Japan’s air forces had been ruined, her navy wrecked. The bulk of Japan’s army was withering away in South Asia.
Nevertheless, on August 6, the first atomic bomb, dubbed “Little Boy,” was dropped on Hiroshima. [Three days] after that, the second atomic bomb, “Fat Man,” was detonated over Nagasaki.”
(Danger’s Hour, The Story of the USS Bunker Hill, Maxwell Taylor Kennedy, Simon & Schuster, 2008, pp. 195-196; page 443)