Hiroshima and Nagasaki
My ten-year research and writing of Imperial Japan's World War Two 1931-1945 convinces me that the US decision to use the A-bomb should be primarily weighed from the perspective of the 14-year Asian-Pacific War it terminated and the mindset of Japan's military leaders who started the merciless tragedy in 1931 and refused to end it in 1945.
Japan, crowded and poor in raw materials but with a sense of military invincibility, saw empire as its salvation and invaded Manchuria in 1931 leading to their invasion of China proper starting in 1937. Indochina was occupied in 1940-41. Japan's imperial regime had volatile ambitions but limited resources, thus encouraging them to unleash a particularly brutal offensive against the people of Asia and the Pacific and Indian Ocean islands.
The League of Nations and the US opposed these moves from the start and increasingly restricted the sale of war materials to the invader from 1938 on. This aroused Japan to attack Pearl Harbor in December 1941 followed by an invasion of resource-rich Southeast Asia. Japan attacked and occupied Malaya, Thailand, Burma, Hong Kong, Singapore, the Philippines, Borneo, the vast East Indies and New Guinea. The Imperial forces captured Pacific and Indian Ocean islands as a defense screen for its new empire. All states attacked, Asian and Western, became allied with the US. The common goal was to protect their populations by defeating Japan as soon as feasible.
It will surprise most Americans to know that Imperial Japan, in fact, directly attacked and occupied a far larger part of the world and its population than did Nazi Germany and her European partners together. Japan caused nearly as many deaths in the East as Germany and her partners did in the West. Atrocities and destruction were in many respects comparable. World War II brought a holocaust, that is a great devastation, a reckless destruction of life, to Asia as well as to Europe.
Japan attacked Asian and ocean island populations brutally attempting to subjugate approximately 20 percent of the world's population. The Chinese-Japanese battles over the span of the war were far greater in number and ferocity than most Americans realize. Typically, the key battles in China involved more troops than engaged in many major WWII battles in Western Europe or in the Pacific. The Asian military and civilian losses were far greater. The battles throughout China and Southeast Asia were at times as devastating to life, cities and towns as those in Russia. Imperial Army campaigns against Chinese city regions of Shanghai, Nanking, Hankow, Changsha, the Kweilin/Canton/Liuchow region and the 1945 battle for Manila, Philippines, each caused at least as many civilian deaths as the atomic bomb dropped on Hiroshima to help end it. Imperial Army "Three alls - kill all, loot all, burn all" anti-guerilla campaigns and other atrocities, including biological warfare, inflicted further massive killing. Forced hard labor brutality, refugee flight rigors, and POW maltreatment grew the death & pain toll.
Destruction wrought by the war and the Japanese occupation policies added to Asia's prewar woes by badly disrupting marginal economies, leading to famine and epidemics. Altogether, the victims of aggression took many forms and were massive in number. Allied civilians, throughout Asia and ocean islands, unquestionably were the major casualties.
US and Allied land and sea forces fighting and dying throughout the Pacific from Guadalcanal to Okinawa, and in New Guinea, the Philippines and Burma made up the war winning struggles that took the war to Japan's heartland, as China defended herself and held down large Japanese forces.
The Chinese suffered about 30,000 deaths per week on average resisting aggression from 1937 to 1945. By August 1945, before the A-bombs were dropped, each added week of war throughout Asia and the Pacific doomed approximately 100,000 Asian and Western Allies plus 50,000 Japanese to death. Up to that time the war had killed close to 24 million Allies plus nearly 3 million Japanese and brought destruction to cities, villages and economies throughout Asia. The Allies suffered approximately 124 million and the Japanese 21 million casualties in dead and surviving, severely affected casualties. The US invasion of Japan loomed ahead with the expected huge loss of American and Japanese lives.
American and Allied leaders who had to deal with this long cruel struggle, and experienced the inconclusive end of the First World War would have had the decisive termination of WW II uppermost on their minds. The Allies had good reason to insist on terms of surrender that stripped Japan's militarists of all influence, and unquestionably demilitarized the nation. Clearly this made concessions to the military impossible and guaranteeing Emperor Hirohito's retention a much more difficult decision than revisionists would allow.
In the full context of the times, I believe that the overwhelming reason the A-bomb was dropped was to quickly and decisively end the war and save lives. The decision undoubtedly spared a million or more lives and many millions more physically and psychologically wounded victims from being added to WWII's toll. City after city in the path of the fighting and bombing were spared further destruction.
It is critical to understand why it took the severe shock of two atomic bombs to end the war. The Japanese militarists' blind adherence to the Bushido and Shinto ethic required their young men to fight to the death without exception. Surrender was not an option! Japan had never lost a war to another nation and had sometimes won against great odds. These leaders clung to the belief in Yamato damashii, Japanese raw courage, which in the end would overcome all Western technological and material advantages. The fates would not allow Japan to lose!
The reality is that the cost in Japanese dead from all US 1945 bombing of their cities to end WWII was only about 2% of all Allied Asian and Western civilian dead from Imperial Japan's long years of aggression. 
When Imperial Japan embarked on war, it caused some 150 times more deaths for the purpose of conquest than did the US by dropping two atomic bombs to end the merciless war. It is surprising that the use of the A-bomb to end the 14-year cataclysmic struggle is a moral question while Japan's responsibility for starting the conflict in the first place, with all its consequences, is not. Unfortunately, the world has lost an unmatched lesson on what starting a war means.
See Imperial Japan's World War Two 1931-1945 Chapter 13: The Bombs of August: Hiroshima and Nagasaki in Perspective for a more complete discussion and data.
- www.japanww2.com Web Table 6, Book p.59 Figure 5.1.
- www.japanww2.com Web Table 9 (bottom)..
- www.japanww2.com Web Table 5, Described throughout the book. Book p.31, p.212 Figure 13.2.
- www.japanww2.com Web Table 5,7 & 11, Book p. 59 Figure 5.1, p.62 Figure 5.2, p.144 Figure 9.2 & 9.3.
The severely affected but surviving casualties include the wounded, maimed, raped and tortured, massive number of refugees, homeless, forced labor, war orphans and widows, plus POWs and internees, Japanese-caused opium addicts, victims of severe war-caused malnutrition and diseases, and the ill-equipped and sometimes ill-cared for Chinese conscripted soldiers and civilian laborers.
Research into these personal tragedies, short of death, clearly demonstrates there were a very high number of victims. The Allied who were severely affected summed to about four times the number of total dead. Since the ratio of military combat wounded to dead was normally about two to three to one in WWII, a four to one ratio for a combined wider range of civilian and military casualties supports the reasonableness of the count. Still, the total is probably conservatively low because the ordeal of the poor communities swamped by destitute refugees was not counted. In any case, the number of surviving casualties, added to count of the dead, provides a stark tabulation of the scale of Asia's WWII suffering.
The Japanese who survived but were badly harmed included the wounded, maimed and some malnourished and diseased, the homeless, widows, orphans, and surviving military conscripts. Their dead plus the badly harmed also came to a high number but was far less than that of the populations they attacked.
- www.japanww2.com Web Table 5 & 11, Book p.144 Table 9.2.
- www.japanww2.com Web Table 5 & 11, Book p. 212 Figure 13.2, Book p.173 Chapter 12.