I have been doing some reading on the end of WWII and it baffles me that there was not a general consensus of disapproval by the international community in regards to these attacks. Of course I understand this was a counterattack after Pearl Harbour, but that was an attack resulting in the death of 2,300 soldiers and less than 100 civilians. While that is abhorrent, it does not seem significant in the context of a world war. However, the way nuclear weapons utterly destroyed large areas of inhabited land, killed hundreds of thousands of civilians and effected the long-term health and quality of life of countless others, it seems even objectively excessive. Through years of history lessons and my own attempt at research, why does the justifiability of this attack never seem to be a topic of discussion?
The atomic bombing of Japan was in no way a "counterattack after Pearl Harbour". The two actions were not militarily directly linked, so the to use the term "counterattack" is wrong.
You were maybe suggesting that the bombings were a "revenge" for Pearl Harbour, but that is also highly debatable. I am not aware that it was a factor in the decision to use the atomic bombs.
In reality the use of the atomic bombs was a way to try to bring to an end the nearly 4 years of fighting in the pacific front, with millions of casualties (8 years if you place the start of hostilities in the pacific theater with the invasion of China by Japan in 1937).
There has been a huge debate over the years about the opportunity of the bombings. But whatever one think about the first (as so far mercifully only) military use of atomic weapons, there are few doubts that the only other way to end the war at the time would have been an amphibian invasion of Japan.
In that event it has been estimated that the casualties numbers, both military and civilian, would have been much higher, and the war would have lasted for many more months or even years.
Actually, during the International Military Tribunal for the Far East (Japan's Nuremberg Trial) one of the justices, Justice Radhabinod Pal, dissented from the other judges, and found Japan not guilty of Class A war crimes, and one of his arguments was that America was guilty of the worst crimes of WW2 (edit: in the Pacific Theater) in the bombings of Hiroshima and Nagasaki.
(He also believed that Japan shouldn't be found guilty of crimes of aggression when colonization by European powers wasn't considered a crime of aggression.)
His lengthy dissent is a fascinating (though very hard to find) read. I believe this page may have all of it.
Source: DISSENTIENT JUDGEMENT OF JUSTICE R.B. PAL, TOKYO TRIBUNAL (Very biased introduction added decades later)
Edit: The dissenting opinion is a 700 page book and is typically only found in law libraries. It is under-published in America, but is well known in Japan, and is often presented in the context of Japanese revisionist history. The link provided has an introduction that is not from the dissenting opinion, but I cannot find this text online anywhere else.
This is a little bit off topic, but in terms of "revenge for Pearl Harbor," the July 1945 air raid on Kure was considered to be exactly that, not the atomic bombings-at least, that's what Admiral Halsey said in his memoirs. Halsey was known for being one of the more fervantly anti-Japanese of the US Naval staff, for what it's worth.
Follow-up question: was Pearl Harbor ever used to justify Hiroshima or Nagasaki? I was under the impression that the assumption of massive casualties in case of an invasion was the main argument (at least towards the public).