I am American and have lived in Japan for several years. I have visited Hiroshima, seen the Atomic Dome and gone to the Memorial Museum. I love this country deeply and respect the culture and people. That said I believe the Hiroshima bombing was necessary. The Nagasaki bomb however was not.
The only valid arguments against the Hiroshima bombing were A, that it was wholesale genocide, and B that Japan was already ready to surrender.
Considering that it was war, the genocide argument is really a moot point. Sure it was civilians, but had World War II continued for just a few weeks more, the sum loss of life would have exceeded the Hiroshima bombing victims. The Japanese were still in China, Korea and South East Asia. They were killing tens of thousands of people a day through ruthless genocide and slave labor, the majority of which were civilians. Moreover, Russia had declared war on Japan and attacked their soldiers in Manchuria and Northern Korea shortly after the bombing. This was planned out far in advance (decided at Yalta convention by the Allies) and the Russians were preparing to move further in to China and the Korean Peninsula which would have opened up an entirely new theater of war and had long lasting ramifications to the people of China and South Korea (More so than the Korean war 5 years later). It was the combination of the bombing and the Russian invasion that forced the Japanese to surrendered. But had it not happened when it did all the killing in Asia would have kept going. Moreover, America would have continued its conventional bombings of major Japanese cities leading to more deaths. Lastly, famine throughout the war torn countries was killing just as many people if not more than military action. In Vietnam and parts of China it was particularly bad. Only after the war ended were the countries able to rebuild their agricultural sector (China though was screwed because they were soon plunged in to a bloody civil war after the close of WWII).
Next is the argument that Japan would have surrendered regardless. People who support this view point out that the Japanese government wanted to surrender only did not know how. This is what caused the political deadlock that occurred once the American Military established airfields within bombing range of Honshu. At that point the Japanese realized their country would be pummled in to submission and starved by bombings, the disruption of shipping supply lines and destruction of agriculture and manufacturing. However, the political brass (the emperor included) were not prepared to surrender because it would have meant that the emperor be removed from power and possibly indicted for war crimes. The terms given by the Allies was for unconditional surrender, so there was no protection or immunity for anyone. The Japanese military leaders were even more extreme in their opinion. Many opposed the thought of surrender and were prepared to fight to the end or at least to the point of an Armistice (Operation Ketsugo). For them that was a much more honorable outcome than being executed by a war crimes tribunal (while the emperor was eventually spared, most Japanese generals were not). And it kept in line with their military policies leading up to that point in the war of death before surrender.
Of course the deadlock was broken by the combination of the atomic bombings and the Russian invasions. But even then when the Emperor declared surrender, resistance in Manchuria by Japanese soldiers continued. The military also attempted a coup to overthrow the emperor and continue the war, one that ultimately failed. Opposition was stifled mostly by a humbling radio address given by the Emperor that states:
... Despite the best that has been done by everyone—the gallant fighting of the military and naval forces, the diligence and assiduity of Our servants of the State, and the devoted service of Our one hundred million people—the war situation has developed not necessarily to Japan's advantage, while the general trends of the world have all turned against her interest.
Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.
Such being the case, how are We to save the millions of Our subjects, or to atone Ourselves before the hallowed spirits of Our Imperial Ancestors? This is the reason why We have ordered the acceptance of the provisions of the Joint Declaration of the Powers.
In the address it clearly states that the Bombings were the main contributing factor to the surrender. While people may argue the the Russian invasions of Manchuria had more of a role to play this did not pose as large or immediate a threat to the main islands. Japan would have fought on trying to reach an armistice than surrender right out, or prolonged a declaration of surrender. That would mean more deaths as mentioned earlier. And had it lead to an actual invasion of Japan (Operation Downfall and the planned Russian invasion of Hokkaido) the amount of lives lost would have been incalculable.
Now for the points in favor of the bombing.
The obvious is that it abruptly ended the war. But more than that the bombings set a precedent in the use of Nuclear arms. After seeing the destruction it cause on a city they were never used again after the end of the war. The powers at be had to see that. They had to understand. And that was only a Uranium bomb. The Hydrogen bombs of today are far more devastating, but humanity is still aware of their destruction in a sense because of what happened in Japan. Otherwise, humans as stupid as they are, would have likely used them at some point down the line with horrific results.
The Hiroshima bombing with the threat of another nuclear attack would have been sufficient enough to achieve the above. The Nagasaki bombing was completely unnecessary. The Americans only did it because they wanted to send more of a message to the Russians, and needed to use Fatman before it became useless due to radioactive decay of its components (Uranium isotope). Better to drop it on civilians than let it go to waste, or so Truman decided. Simply disgusting.
