From Tennozan: The Battle of Okinawa and the Atomic Bomb, by George Feifer (Ticknor & Fields, 1992), pp. 571-574:
American anticipation of the bloodbath [awaiting them when they invaded the main islands of Japan] was evident in the forty-two divisions they allotted to the invasion. Seven had fought on Okinawa.
The planners calculated the landing alone would cost a hundred thousand American lives. The full securing of the home islands was expected to cost ten times that number, or four times the combined losses of Hiroshima and Nagasaki. General MacArthur, whose estimates of casualties in previous battles had been uncannily accurate, made a careful study of the mainland operation at President Truman’s request and predicted one million men would be killed or wounded in the invasions of Kyushu and Honshu alone…. Final victory might easily cost more American casualties than in the entire war until then, in both the Atlantic and Pacific theaters.
The predictability of the [Okinawa] veterans’ renewed love of the bomb when they saw what it saved them from at mainland landing sites is no reason to dismiss arguments for its use. Of course it killed many people, but the equation, if there is one, must include the people it saved, to the extent that saving now seems likely and the number can be estimated. Although the American fighting men who cheered Little Boy and Fat Man did not care as much about others’ survival as their own, consideration of the larger issue must include possible Japanese losses.
The ratio of Japanese combat deaths to American was well over 10 to 1 on Okinawa. It might have been marginally different during fighting in the enemy’s heartland rather than on isolated islands, where Japanese garrisons were often cut off from reinforcements. Civilian deaths assuredly would have been much higher, if only because the mainland had many more civilians with a commitment to die for Emperor and country. The best estimates of probably total Japanese deaths in a mainland campaign are around twenty million; if civilian suicides and suicidal resistance had generated hysteria – a likely prospect in light of the experience on Guam [sic; Saipan?] and Okinawa – the toll would have been higher. The country would have been leveled and burned to cinders. Postwar life, including economic recovery, would have been retarded if Russia, a full Allied partner during the ground combat from 1945 to 1947 or 1948, would have insisted on dividing Japan like Korea and Germany.
Any estimate of lives saved by the atomic bombs must include hundreds of thousands of combatants and civilians in China, Manchuria and other territories still fought for and occupied, often viciously, by Japan. There would have been tens of thousands of British casualties among the 200,000 set to invade the Malay Peninsula – to retake Singapore – on September 9, a month after Nagasaki. Six divisions, the same number as at Normandy, had been assigned to that operation. It was expected to take seven months of savage infantry fighting, over half the time required to defeat Hitler’s armies in Europe.
The total number must also include European and Eurasian prisoners of the Japanese, chiefly from English, Dutch, and other colonial military and civilian forces. Okinawa was the most important prelude to the climax because its terrain most closely resembled the mainland’s, but non-Japanese elsewhere in Asia would have suffered even more during the new Tennozan. After the fall of Okinawa, Field Marshal Count Hisaichi Terauchi issued an order directing his prison camp officers to kill all their captives the moment the enemy invaded his southeast Asia theater. That would have been when those 200,000 British landed to retake Singapore, less than three weeks after the Japanese surrender. There was a real chance that Terauchi’s order would have been carried out, in which case up to 400,000 people would have been massacred. Even more were doomed to die soon after of “natural” causes. The Japanese treatment of their prisoners grew more brutal as the military situation worsened and their hatred swelled. Laurens van der Post, who had been a prisoner for more than forty months, was convinced that the majority of the half-million captives in the hellish camps could not possibly have survived the year 1946. Dying every day in droves throughout the summer of 1945, nearly all would have perished of disease and starvation in the months that followed.