Barack Obama visited Hiroshima on Friday, the site of the world's first atomic bombing, in the first visit to the Japan city by a sitting US president.
He lay a wreath of white flowers in tribute to the victims of the bombing on August 6, 1945 which killed thousands of people instantly and 140,000 by the end of the year.
The President's visit is widely seen as a symbolic gesture that Tokyo and Washington hope will highlight their alliance and breathe life into efforts to abolish nuclear arms.
After laying the wreath at the Hiroshima Peace Memorial and paying his respects, Obama said: "71 year ago on a bright cloudless morning, death fell from the sky and the world was changed. A flash of light, a wall of fire, destroyed a city and demonstrated that mankind possessed the means to destroy itself."
Aides said Obama's main goal in Hiroshima is to showcase his nuclear disarmament agenda, which won him the Nobel Peace Prize in 2009.
That is why we come to this place, we stand here in the middle of this city and force ourselves to imagine the moment the bomb fell. We force ourselves to feel the dread of children confused by what they see. We listen to a silent cry.
A message left in the Hiroshima Peace Memorial guest book by the President read: "We have known the agony of war. Let us now find the courage, together, to spread peace, and pursue a world without nuclear weapons."
Before leaving the Memorial, President Obama greeted a few survivors of the bombing 71 years ago.
Survivor Shigeaki Mori, looked visibly moved as he met the US president.
The 77-year-old local historian has spent 20 years tracking down the families of American POWs who were killed when the bomb dropped in Hiroshima to honour their memory.
A majority of Americans see the bombings of Hiroshima and Nagasaki, which was hit by a second bomb three days later, were necessary to end the Second World War but some historians question that view.
Before his visit, Obama said he will honour all who died in the Second World War but he would not apologise for the bombing.
Obama's visit has stirred debate, with critics accusing both sides of being contradictory by calling for an end to atomic arms whilst relying on nuclear deterrence.