In 1945, World War Two - the biggest war the world had ever seen - was coming to an end.
There were celebrations in Europe after Germany surrendered.
But on the other side of the world in the Pacific Ocean, Japan was still fighting against the US, Britain and their allies.
The Americans, however, had a secret plan to end the war for good - by using the most powerful weapon ever created.
On 6 August 1945 at 8.15am Japanese time, an American B-29 bomber plane called Enola Gay dropped an atomic bomb on the Japanese city of Hiroshima.
The devastation was unlike anything in the history of warfare. The city was immediately flattened.
Around 80,000 people were killed as a direct result of the blast, and another 35,000 were injured.
Even after this devastation, Japan did not surrender.
Three days later, another nuclear bomb was dropped by the Americans on the Japanese city of Nagasaki.
Shortly afterwards, on 15 August 1945, Japan finally admitted defeat and World War Two was over.
When the bomb exploded in Hiroshima, the city was struck by a flash of blinding light, then a giant cloud shaped like a mushroom rose into the sky.
The blast flattened buildings within a 2.5 km radius of the bomb.
There were 90,000 buildings in Hiroshima before the bomb was dropped, but only 28,000 remained after the explosion.
Thousands and thousands of people were killed. Many were badly injured.
But the suffering didn't end there because it wasn't just a normal bomb.
The nuclear radiation released when it exploded caused people to suffer horrible illnesses.
Thousands more people died from their injuries and radiation sickness in the weeks, months and years that followed.
Japan was at war with America and its allies, which included Britain and Soviet Union - a nation made up of modern-day Russia and other countries.
The allies were winning the war and the Japanese forces had been pushed back from many locations.
However, fighting was still very fierce, and soldiers and civilians were dying every day.
Japan had been at war for many years.
It had invaded the countries near to it such as China and the Japanese had attacked America.
Everywhere the Japanese soldiers went, they were known for their cruelty.
They treated prisoners of war very badly, including American and British soldiers who had surrendered.
US President Harry S Truman wanted the Japanese to surrender as quickly as possible so he could save lives.
The atomic bomb was a deadly new weapon and President Truman hoped the massive destruction it caused would shock the Japanese into realising they had to surrender.
US President Truman wanted to avoid a land invasion of Japan.
There were 2.5 million Japanese troops stationed there and Truman's staff estimated that defeating them would cost the lives of 250,000 US soldiers.
Some historians also say that the US wanted to avoid Japan being occupied by Soviet troops.
America and the Soviet Union were allies but they did not really trust each other.
It was the first and only time that atomic bombs have been used in a war.
Although the scientists who made the bombs were proud of what they'd achieved, it scared them as well.
The way the atomic bomb was built meant it had huge power - enough to destroy whole cities on its own.
Many people now believe that the devastation caused in Hiroshima and in Nagasaki was so awful that the bombs should never be used again.
Today, a small number of countries around the world, including the US, China and the UK, have nuclear weapons.
Some campaigners argue that there is no place for nuclear weapons and that all countries should get rid of them immediately.
Others say that having such terrible weapons will help to keep a country safe, even if they are never used.