Today, 6 August 2020, is the 75th anniversary of the dropping of the first nuclear bomb which devastated the lives of hundreds of thousands. Ethical, legal and military controversies still surround this and the bombing of Nagasaki three days later.
Winston Churchill argued a few days after the bombing that “There are voices which assert that the bomb should never have been used at all. I cannot associate myself with such ideas. … I am surprised that very worthy people—but people who in most cases had no intention of proceeding to the Japanese front themselves—should adopt the position that rather than throw this bomb, we should have sacrificed a million American and a quarter of a million British lives.”
On 26 July, twelve days before the Hiroshima bomb, the President of United States, Prime Minister of the United Kingdom and Chairman of the Chinese Nationalist Government had issued the Potsdam Declaration which outlined the terms of surrender for the Empire of Japan. This ultimatum stated if Japan did not surrender, it would face “prompt and utter destruction”. But it was clear at the time that Japan would never accept the terms of that declaration.
But would Japan have surrendered even without the bomb? On the day of the Hiroshima bomb, the Japanese ambassador in Moscow was sounding out the Soviets on the terms for a negotiated end to the war. The destruction of Hiroshima did not change the Japanese negotiating position in the days that followed.
On the evening of 8 August, the Soviets announced they would be entering the war against Japan, as Stalin had promised Franklin Roosevelt and Churchill at Yalta. Hours after that, the second atomic bomb was dropped on Nagasaki.
Historians are divided over whether the Soviet declaration alone might have ended the war.
In an analysis published yesterday in the Guardian , Julian Borger reported Tsuyoshi Hasegawa, former research professor at the University of California in Santa Barbara, and an expert on Soviet-Japanese diplomacy at the time as saying:
“Despite the Hiroshima bomb, the Japanese government still continued to seek the termination of the war through Moscow’s mediation. I would say that the Soviet entry into the war had a more decisive impact on the decision to surrender than the atomic bombs.”
A view disputed by others.
Further analysis of the many issues raised on both sides of the argument about the justification for the bombing can be found in Wikipedia  and the many references it contains to further discussion.