Guide: What happened in Hiroshima?

Last updated at 05:36
The plume of smoke from a mushroom cloud billow, about one hour after the nuclear bomb was detonated above Hiroshima, JapanAP
A huge mushroom cloud of smoke fills the sky over Hiroshima, Japan, one hour after it was bombed in August 1945

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.

What happened?

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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.

In this undated picture provided by the U.S. Strategic Bombing Survey, the destroyed city of Hiroshima, Japan is seen from the Red Cross hospital building located about one mile from the epicenter of the nuclear blast from the 'Little Boy' atomic bomb which was detonated by the U.SAP
The atomic bomb destroyed thousands of buildings in Hiroshima. The city was left flattened

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?

This building, now called the A Bomb Dome, survived the atomic blast and today it's a memorialEPA
This building, now called the A Bomb Dome, survived the atomic blast and today it's a memorial in Hiroshima, Japan

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.

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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?

The Hiroshima bomb was nicknamed 'Little Boy'. Here it is before being loaded into the Enola Gay's bomb bayPublic Domain
The Hiroshima bomb was nicknamed 'Little Boy'. Here it is before being loaded into the Enola Gay's bomb bay

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.

A Japanese kamikaze pilot tying on the honorary ribbons that were always worn when on a suicide mission. 'Kamikaze', which means 'winds of the gods', were pilots who deliberately crashed their bomb laden planes onto American warships during the Second World War.Getty Images
This Japanese Kamikaze pilot is about to set off and deliberately crash his bomb-packed plane onto American warships during World War Two

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.

Children in Hiroshima, Japan, wearing masks to combat the odour of death after the city was destroyed by the first atom bomb, October 1945Getty Images
Children in Hiroshima, Japan, wearing masks to protect themselves from the polluted air caused by the bombing

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.

American President Harry S Truman (1884 - 1972) announcing that the Soviet Union is joining the United States, Great Britain and other United Nations in the war against Japan.Getty Images
Harry S Truman was the US President during World War 2 and it was his decision to drop the nuclear bomb on Hiroshima

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?

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Leah looks around Hiroshima's Peace Memorial Park

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.

Tsuyuko Nakao (R), 92, and Kinuyo Ikegami, 77, survivors of the atomic bombing, comforting each other after offering prayers for victimsEPA
Two survivors of the atomic bombing, Kinuyo Ikegami (left) and Tsuyuko Nakao (right), hug each other after offering prayers for victims at a ceremony in 2010

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.