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 America, Britain and their allies.
The Americans, however, had a secret plan to end the war - 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 beyond anything seen before. The city was immediately flattened.
80,000 people were killed as a direct result of the blast, and another 35,000 were injured.
Even then, Japan didn't 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.
World War Two was over.
What damage did the bombs cause?
When the bomb exploded in Hiroshima, the city has struck by a flash of blinding light then a giant cloud shaped like a mushroom.
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.
Why did the US drop the bomb?
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.
President Truman hoped the massive destruction it caused would shock the Japanese into realising they had to surrender.
What did the US hope to avoid?
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.
Why is this event so important?
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 it's 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 USA, 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 keep a country safe, even if they are never used.