(USA TODAY) -- When Franklin Roosevelt died on April 12, 1945, Harry Truman suddenly found himself president — without a clue that he would soon have to make a decision that would change the world.
Less than four months later, President Truman unleashed the nuclear age, ordering that atomic bombs be dropped on the Japanese cities of Hiroshima and Nagasaki, killing as many as 150,000 people.
Truman never expressed regret for the first — and so far only — use of nuclear weapons in wartime. He argued that by bringing about the swift surrender of Japan, and the end of World War II, it prevented an even more fearsome death toll from an Allied invasion of Japan.
"It was a terrible decision. But I made it," Truman wrote to his sister, Mary. "And I made it to save 250,000 boys from the United States, and I'd make it again under similar circumstances."
The world has had its share of nuclear anxiety in the seven decades since Truman's decision, from Cold War confrontations such as the Cuban missile crisis to modern fears about terrorists getting their hands on atomic weapons.
During that time, Truman has come in for criticism from some historians who say Japan could have been induced to surrender in other ways.
"In many parts of the world, this is regarded as horrible atrocity," says Gar Alperovitz, who has written critically of the Hiroshima and Nagasaki bombings. He says Truman's decision was unnecessary.
There's little evidence, however, that Truman ever considered an alternative to using the atomic bomb.
As the first months of his presidency coincided with the final development of the bomb, only one major question loomed: Would it work? After that, it was just a matter of when and where to use it.
For Truman, the process began just minutes after being sworn in. Secretary of War Henry Stimson pulled the new president aside with a cryptic message, Truman wrote in his memoirs: "He wanted me to know about an immense project that was underway, a project looking to the development of a new explosive of almost unbelievable destructive power."
Days later, Stimson gave Truman more details about what had been dubbed the Manhattan Project: "Within four months, we shall in all probability have completed the most terrible weapon ever known in human history."
The prospect of a history-altering bomb that could kill hundreds of thousands at once came as Truman faced a rush of other unprecedented presidential challenges.
Among them: managing the end of the war with Germany, helping to form the United Nations and preparing to meet with Soviet leader Joseph Stalin and British Prime Minister Winston Churchill about the fate of the postwar world.
Uncertainty about whether the bomb would work — and there were plenty of skeptics — prompted Truman and his aides to plan for an invasion of Japan that would start in November 1945.
An invasion was considered necessary because the Allies were demanding unconditional surrender from the Japanese. But the Japanese military — led by fanatical commanders, fighting for an emperor they considered a god on Earth — seemed unlikely to give up without an epic battle.
The risks of an invasion put pressure on U.S. planners to complete the design of the atomic bomb.
On July 16, 1945, as Truman attended a summit with Stalin and Churchill at Potsdam, Germany, the president received a coded telegram about an atomic test. "Operated on this morning," it read. "Diagnosis not yet complete but results seem satisfactory and already exceed expectations."
Follow-up tests confirmed: The atomic bomb would work.
Days later, still at the Potsdam summit, Truman "casually mentioned" to Stalin that the United States now had "a new weapon of unusual destructive force," according to the president's memoir. Stalin said he hoped it would be put to "good use" against Japan.
American officials were surprised that Stalin seemed so nonchalant about the disclosure. They later learned why: Soviet spies had kept him apprised of U.S. work on the atomic bomb. The Soviet Union tested its own nuclear bomb in 1949.
The evidence suggests Truman made his decision to use the atomic bomb on July 24, also at Potsdam.
For Alperovitz and other critics, the question is not so much how Truman made his order to bomb Japan, but why.
One theory is that Truman wanted to knock Japan out of the war before the Soviet Union had time to join an invasion, denying it a chance to exercise more influence in the region. Other theories: The simple desire for revenge against the Japanese, and the sheer momentum of the bomb-building effort. FDR diverted $2 billion in war budgets to the project, and there is little doubt he would have dropped it on Japan (and Germany, for that matter).
Alonzo Hamby, a biographer of Truman, says the president who had to make the call had one overriding concern: "Winning the war and getting it over with as soon as possible," especially given the prospects of American deaths in an invasion.
Weighing heavily on American minds at the time: The bloody invasion of the island of Okinawa that lasted 82 days and claimed more than 12,500 U.S. lives. The military estimated that an invasion of the Japanese home islands could cost 250,000 or maybe even 500,000 American lives.
Using the atomic bomb? "I'm not sure much went into the thinking at all," Hamby says.
The B-29 bomber Enola Gay dropped the first atomic bomb on Hiroshima on Aug, 6, 1945, as Truman sailed back from Europe aboard the USS Augusta.
"Results clear-cut — successful in all respects," read a message Truman received as he ate lunch.
In a speech to the nation that week, Truman said he realized "the tragic significance of the atomic bomb." He also spoke about the awful deaths of women and children in Japan. "All those kids," he told his Cabinet after Nagasaki was hit.
But he never second-guessed his basic decision, at least not publicly.
In that letter to his sister, Truman said he couldn't worry about what history might say about his personal morality: "I made the only decision I ever knew how to make. I did what I thought was right."
