Obama's trip to Hiroshima made him the first US president to visit the site of the world's first atomic bomb attack, and he sought to walk a delicate line between honouring the dead, pushing his as-yet unrealised anti-nuclear vision and avoiding any sense of apology for an act many Americans see as a justified end to a brutal war that Japan started with a sneak attack at Pearl Harbor.
"The flash of light and a wall of fire destroyed a city and demonstrated that mankind possessed the means to destroy itself."
In a carefully choreographed display, Obama offered a somber reflection on the horrors of war and the dangers of technology that gives humans then "capacity for unmatched destruction."
With Japanese Prime Minister Shinzo Abe standing by his side and an iconic bombed-out domed building looming behind him, Obama urged the world to do better.
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A second atomic bomb, dropped on Nagasaki three days after Hiroshima, killed 70,000 more. Japan surrendered on Aug. 15, 1945, ending a war that killed millions.
Obama hoped Hiroshima would someday be remembered not as the dawn of the atomic age but as the beginning of a "moral awakening." He renewed his call for a world less threatened by danger of nuclear war.
"Among those nations like my own that hold nuclear stockpiles, we must have the courage to escape the logic of fear and pursue a world without them," Obama said.
Abe, in his speech, called Obama's visit courageous and long-awaited. He said it would help the suffering of survivors and he echoed the anti-nuclear sentiments.
"At any place in world, this tragedy must not be repeated again," Abe said.