Barack Obama is the first serving American President to visit Hiroshima in Japan. It is the city which was destroyed by a US atomic bomb in 1945.
The bombing - and a second one on the Japanese city of Nagasaki three days later - is credited with bringing to an end World War Two but it claimed the lives of hundreds of thousands of people.
Survivors of the attack were at a ceremony with President Obama where he honoured all those who lost their lives in World War Two.
An American B-29 bomber called the Enola Gay dropped the nuclear bomb, exploding some 600 metres above the city on 6 August 1945.
On that day alone, at least 70,000 people are believed to have been killed.
Thousands more people died from their injuries and radiation sickness in the weeks, months and years that followed.