Barack Obama has arrived in Hiroshima to become the first serving US president to visit the Japanese city since the 1945 nuclear bombing.
Mr Obama flew into the Iwakuni US base nearby, after leaving the G7 summit.
He said his visit was “a testament to how even the most painful of divides can be bridged”. But he also says he will not be apologising for the attack.
At least 140,000 people died in Hiroshima on 6 August 1945, in what was the world’s first nuclear bombing.
Two days later a second nuclear bomb was dropped on Nagasaki, killing another 74,000.
Television footage showed Mr Obama arriving and entering the Hiroshima Peace Memorial Museum.
He then walked to the Peace Memorial Park, accompanied by Prime Minister Shinzo Abe. Both men stood in front of the eternal flame.
Mr Obama laid his wreath first, followed by Mr Abe.
Mr Obama earlier told service personnel at the Iwakuni Marine Corp base, some 40km (25 miles) from Hiroshima: “This is an opportunity to honour the memory of all who were lost during World War Two.
“It’s a chance to reaffirm our commitment to pursuing the peace and security of a [world] where nuclear weapons would no longer be necessary.”
Mr Obama praised the US-Japan alliance as “one of the strongest in the world”, with his visit showing how “two nations, former adversaries, cannot just become partners, but become the best of friends”.