WASHINGTON (AFP) - Barack Obama's visit to Hiroshima next week has reignited an emotive debate over former US president Harry Truman's epoch-making decision to drop the first atomic bomb.
On April 25, 1945, 13 days after Franklin Roosevelt's death thrust Truman into the White House, the strained new commander-in-chief got a startling top secret briefing.
"Within four months we shall in all probability have completed the most terrible weapon ever known in human history, one bomb of which could destroy a whole city," war secretary Henry Stimson said in a hand-delivered memo.
Until that moment, Truman had no idea about the Manhattan Project to build the world's first atomic bomb - despite being Roosevelt's vice president and a former senator who made his name investigating wartime defence contracts.
Within four months, the atomic bomb had been successfully tested, targets had been selected, "Little Boy" and "Fat Man" had been dropped on Hiroshima and Nagasaki killing an estimated 214,000 people, and Japan's Emperor Hirohito had surrendered.
The speed, circumstances and repercussions of Truman's decision remain contentious.
That is true not least in Japan, where a majority of Obama's hosts still believe the mass bombing of civilians was unnecessary and perhaps even a crime.
Meanwhile commentators nervous that Obama's trip is tantamount to an admission of guilt, have urged him not to apologize.
"When Mr Obama visits Hiroshima on May 27 he should place no distance between himself and Harry Truman," wrote Wilson Miscamble, a Notre Dame University history professor.
"Rather he should pay tribute to the president whose actions brought a terrible war to an end." For Truman's supporters, "Give 'em hell Harry" had little option.
By late Spring 1945, American and Russian forces had met at the Elbe, Adolf Hitler was surrounded and the war in Europe was finally ending.
But the Pacific was exacting an ever bloodier toll. Japan showed no signs of surrender, despite heavy losses and a seemingly inevitable defeat.
According to historian and biographer David McCullough, at that point not a single Japanese unit had surrendered during the war.
For Truman, a veteran of the Great War, the bomb, first and foremost, appeared to offer a way out of a brutal ground invasion of Japan.
"Operation Downfall," as the mainland invasion was dubbed, could have involved at least one million US troops and as many as 2.5 million Japanese troops.
With recent battles in Okinawa and Iwo Jima fresh in mind, US military planners believed the operation would cost a quarter of a million lives and extend the war by a year or more.
At the end of July, with the bomb now successfully tested, Truman gave Japan one last chance.
Meeting with Joseph Stalin and Winston Churchill at Potsdam, the three leaders called for Tokyo to "surrender unconditionally" or face "prompt and utter destruction." The allies waited eagerly for a response, which was given by Japanese premier Kantaro Suzuki.
"Mokusatsu", he said when asked by reporters, using a word that would become infamous.
The phrase can mean "no comment", but in this instance was translated as "not worthy of comment."
"US officials, angered by the tone of Suzuki's statement and obviously seeing it as another typical example of the fanatical Banzai and Kamikaze spirit, decided on stern measures," a National Security Agency report on the dangers of mistranslation later noted.
Within Truman's inner circle there were voices against using the bomb, including Dwight Eisenhower, the future president who was then a wartime general.
"I was one of those who felt that there were a number of cogent reasons to question the wisdom of such an act," he later wrote.
But there is little evidence that Truman ever seriously considered forgoing the fruits of a US$2 billion programme that Roosevelt had nurtured in secret for years.
More likely he saw the weapon as a terrible, but useful extension of already terrible conventional weapons.
Neither did there appear to be a recognition that dropping the bomb would catalyse an arms race with the Soviet Union that would define the next half century.
"What American leaders did discuss extensively, and sometimes heatedly, were the questions associated with how, where and when to use the bomb," wrote historian Sean Malloy.
"Should it be used against Germany or Japan? What targets within those countries might be appropriate for such a weapon? Should there be a warning or demonstration first?"
When the bomb was dropped, Truman made little immediate mention of civilian casualties and a few days later even described Hiroshima as a "military base", spurring questions about whether he realised the scale of destruction.
But the White House is quick to scotch suggestions that Obama will revisit the broader issue of whether the bomb should have been dropped at all.
Asked whether Obama would make the same decision as Truman, aide and spokesman Josh Earnest said "I think what the president would say is that it's hard to put yourself in that position from the outside."
"I think what the president does appreciate is that president Truman made this decision for the right reasons. President Truman was focused on the national security interests of the United States... on bringing an end to a terrible war. And president Truman made this decision fully mindful of the likely human toll.
"I think it's hard to look back and second-guess it too much."