On the 60th anniversary of the destruction of Hiroshima, new questions are being asked about whether it was necessary to drop the atomic bomb - and whether the bomb was really responsible for the Japanese surrender.
Historians will never fully agree on the answers.
Would more lives would been lost if the bombings hadn't happened?
And the children born of fathers who might otherwise have been sent to invade Japan in 1945 often wonder if they should not be grateful that the bomb was used, first on Hiroshima on 6 August and then against Nagasaki.
Churchill likened the explosions at the time to the "Second Coming in wrath".
US President Harry Truman also recognised their significance.
When he was told of the successful test of the atomic bomb - and then took the decision to use it with no warning - he wrote in his diary: "We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark."
The opinions of mankind since then continue to be divided as to why the decision was taken.
The orthodox view is that Truman dropped the bomb because the only real alternative was an invasion of Japan.
A continuation of the fire-bombing campaign would not, it was held, bring about a Japanese capitulation because the Japanese army was ready to fight to the end.
Foremost in Truman's mind was the prospect of huge American losses.
Truman wrote: "I asked General Marshall what it would cost in lives to land on the Tokyo plain and other places in Japan. It was his opinion that such an invasion would cost at a minimum a quarter of a million American casualties."
In his biography of Truman, David McCullough says that plans for an invasion were real.
Truman at the Potsdam Conference where he decided to use the bomb
"Nor, it must be stressed, was there anything hypothetical about preparations for the invasion - on both sides - a point sometimes overlooked in later years," he wrote.
"Truman had earlier authorised the Chiefs of Staff to move more than one million troops for a final attack on Japan. Japan had some 2.5 million regular troops on the home islands."
But was enough done to try to negotiate a Japanese surrender?
The Allied position was that Japan had to surrender unconditionally, as Germany had.
A former US ambassador in Tokyo, Joseph Grew, recommended that it should be made clear to the Japanese that they could still keep their emperor.
This, he felt, would enable negotiations to proceed and maybe even succeed.
But the furthest the Allies went, in a declaration issued at Potsdam, was to offer the Japanese people the right to choose their government.
This implied that the emperor could remain, but the language did not state this specifically, and simply said Japan must have "a peacefully inclined and responsible government".
The declaration was ignored by the Japanese government, which was itself divided.
Even the supporters of negotiations could not agree on the terms and the hardliners kept on adding more demands.
Now, a new book offers the most radical re-interpretation of these events. In Racing the Enemy, Tsuyoshi Hasegawa, professor of history and director of the Center for Cold War Studies at the University of California, Santa Barbara, blames both Stalin and Truman for not doing more to negotiate a surrender.
He also claims that it was the Soviet entry into the war against Japan, just after Hiroshima, which really worried the Japanese and which made them give up.
Mr Hasegawa says that Stalin rejected peace feelers put out by Japan because he was determined to win spoils from joining the war.
And, he suggests, the Americans ignored the feelers - which they knew about from breaking Japanese codes - because they did not like them.
Truman refused to modify the "unconditional surrender" demand because he wanted revenge for Pearl Harbor, courted popularity at home and needed to demonstrate strategic power.
Thus, Mr Hasegawa claims, opportunities were lost. The myth that it was only the atom bomb which could have ended the war was invented in order to assuage "Truman's conscience and ease the collective American conscience".
Mr Hasegawa argues that the hard-line Japanese leaders were not overly concerned about the destruction caused by the atom bombs since American conventional bombers could cause the same - and indeed worse - damage anyway.
War Minister Korechika Anami contemplated defeat with equanimity and compared the potential destruction of Japan to the withering of a flower.
What alarmed the Japanese, Mr Hasegawa says, was the Red Army. According to this theory, Japan gave up because it could not accept that Soviet troops might take part in an invasion and occupy part of the homeland, given the history of conflict with Russia in the past.
This interpretation goes against the version as expressed, for example, by American historian Richard B Frank in his 1999 book, Downfall.
Mr Frank concluded: "It is fantasy, not history, to believe that the end of the war was at hand before the use of the atomic bomb."
He examined the effort by the Japanese Foreign Minister Shigenori Togo to open a negotiation with Moscow. This, he pointed out, was so feeble and uncertain that it was even scorned by Japan's ambassador in Moscow, Naotake Sato.
Sato wrote a scathing series of telegrams to his superiors highlighting the empty nature of the Japanese offer. The Americans knew how empty all this was from reading the exchanges between Togo and Sato.
As for the potential impact of an offer to retain the emperor, Mr Frank argues, Foreign Minister Togo himself told Ambassador Sato that an interpretation of surrender terms to include such an assurance would not be enough.
Indeed, ministers like Anami were adding conditions to the end, including a refusal to have Japan occupied at all.
At least 120,000 people died in the atomic bombings outright
Mr Hasegawa's view that it was the Russians, not the bomb, that forced the surrender is also unlikely to be accepted by the traditionalists.
This is because, while it is true that the bombs did not persuade Anami and his cohorts to give up arguing for resistance, they did lead to the crucial intervention of Emperor Hirohito.
He mentioned them in his decisive address to his Cabinet, so they certainly had an effect on him. And it was that address that brought even Anami to heel, though the war minister contemplated a coup and duly killed himself soon afterwards.
The arguments will go on. At a conference organised by Greenpeace in London to mark the 60th anniversary, Professor Mark Selden of Binghamton University in New York argued that strategic considerations lay behind Truman's decision.
"There was a belief that dropping the bomb could accelerate the end of the war in ways that would greatly strengthen the American strategic position in Asia," he said.
"This was in fact a race with the Russians. The bomb was to announce to the world American superiority. It would also stop any Russian advance against Japan and create a situation, as happened, in which the US would dominate the occupation of Japan."
David McCullough argues for a more down-to-earth interpretation of Truman's motives.
"How could a president, or the others charged with responsibility for the decision, answer to the American people if... after the bloodbath of an invasion of Japan, it became known that a weapon sufficient to end the war had been available by midsummer and was not used?"