President Barack Obama is set to arrive in Hiroshima and become the first serving US president to visit Japan’s city since the 1945 nuclear bombing.
Barack Obama flew into a US base nearby, after leaving the G7 summit.
He has said he will not be issuing an apology for the nuclear attack, but will honor all those who died in World War Two.
Barack Obama told Japanese media the visit would show that “even former adversaries can become the strongest of allies”.
The world’s first nuclear bomb attack, on Hiroshima on August 6, 1945, killed at least 140,000 people. Two days later a second nuclear bomb was dropped on Nagasaki, killing another 74,000.
Barack Obama will lay a wreath at the cenotaph, where an eternal flame remembers Hiroshima’s dead. He will be joined by bomb survivors living in the now thriving city.
Many in the US believe the use of the nuclear bomb, though devastating, was right, because it forced Japan to surrender, bringing an end to World War Two.
Jimmy Carter has visited Hiroshima, but after the end of his presidency.
A US ambassador attended the annual commemoration for the first time in 2010.