As Japan marks 75th anniversary of the nuclear bomb attack, a look at how it happened, the devastation it wrought.

The city that survived the world’s first atomic bombing marked the 75th anniversary of the attack in Japan on Thursday, with Hiroshima Mayor Kazumi Matsui urging nations to reject self-centred nationalism and commit more seriously to nuclear disarmament.

In a ceremony sharply downsized due to the coronavirus pandemic, survivors, their relatives and officials stood for a moment of silence as cicadas shrilled in the heavy summer heat and the Peace Bell rang out over Peace Park.

“On August 6, 1945, a single atomic bomb destroyed our city. Rumour at the time had it that ‘nothing will grow here for 75 years’,” Matsui said at the memorial ceremony.

“And yet, Hiroshima recovered, becoming a symbol of peace.”

The mayor went on to urge Japanese Prime Minister Shinzo Abe’s government to sign the international treaty banning nuclear weapons, and called on the world to unite in the face of global threats.

“When the 1918 flu pandemic attacked a century ago, it took tens of millions of lives and terrorised the world because nations fighting World War I were unable to meet the threat together,” Matsui said.

“A subsequent surge in nationalism led to World War II and the atomic bombings. We must never allow this painful past to repeat itself. Civil society must reject self-centred nationalism and unite against all threats.”

Below we take a look at the events that devastated Hiroshima and ushered in the era of weapons of mass destruction.

What happened in Hiroshima?

On August 6, 1945, at about 8:15am local time, the US aircraft Enola Gay dropped an untested uranium-235 gun-assembly bomb nicknamed “Little Boy” over Hiroshima.

The destruction was unlike anything in the history of warfare.

Hiroshima was immediately flattened. The resulting explosion killed 70,000 people instantly; by December 1945, the death toll had risen to some 140,000.

The radius of total destruction was reportedly 1.6km.

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“The impact of the bomb was so terrific that practically all living things – human and animal – were literally seared to death by the tremendous heat and pressure set up by the blast,” Tokyo radio said in the aftermath of the explosion, according to a report by The Guardian in August 1945.

“All the dead and injured were burned beyond recognition. Those outdoors were burned to death, while those indoors were killed by the indescribable pressure and heat.”

But the damage did not end there. The radiation released from the explosion kept causing suffering.

Thousands more died from their injuries, radiation sickness and cancer in the years that followed, bringing the toll closer to 200,000, according to the Department of Energy’s history of the Manhattan Project.

Why did the US drop the bomb?

Japan was a fierce enemy of the US and its allies, Britain, China and the Soviet Union during World War II.

By 1945, the allies had turned the tide of the war and pushed the Japanese forces back from many locations.

The Japanese had publicly stated their intent to fight to the bitter end, and were using tactics such as kamikaze attacks, suicide attacks by Japanese fighter pilots against US warships.

In July 1945, US President Harry Truman and allies demanded the “immediate and unconditional” surrender of Japan, but Japan did not issue a clear response.

Shortly after, the US attacked Hiroshima, chosen because it was seen as a “strategically sound” target based on calculations around weather conditions, aircraft range, military impact and the impact on “enemy morale”.

“With this bomb we have now added a new and revolutionary increase in destruction to supplement the growing power of our armed forces,” Truman said 16 hours after the atomic bomb was dropped.

“We are now prepared to obliterate more rapidly and completely every productive enterprise the Japanese have above ground in any city. We shall destroy their docks, their factories, and their communications. Let there be no mistake; we shall completely destroy Japan’s power to make war.”

Truman said if Japan’s leaders “do not now accept our terms they may expect a rain of ruin from the air, the like of which has never been seen on this Earth”.

What happened after Hiroshima?

After the bomb obliterated Hiroshima, the Japanese did not surrender.

Three days later, the US launched another mission to bomb Kokura. However, after finding Kokura obscured by clouds, Nagasaki was chosen as a target instead.

Based on that, “Fat Man” was dropped over Nagasaki on August 9, 1945, instantly killing at least 40,000 people.

The British pilot Leonard Cheshire, who was involved in the mission to bomb Nagasaki, later recalled the cloud caused by the atomic blast: “Obscene in its greedy clawing at the earth, swelling as if with its regurgitation of all the life that it had consumed.”

The bombings were as questionable back then as they are today. Six out of seven five-star US generals and admirals at the time felt there was no need to drop the bomb because Japanese surrender was imminent.

The nuclear bombing of Nagasaki during World War II [Getty Images]

What was the impact of Hiroshima and Nagasaki?

On August 15, 1945, Emperor Hirohito announced Japan’s surrender, and on September 2, the surrender was formally signed, bringing the hostilities of World War II to a close.

The power of the atomic bomb would usher a change in geopolitics that reverberates to this day, as countries raced to acquire this destructive technology.

The Doomsday Clock

In 1947, the scientists involved in the Manhattan Project created the Doomsday Clock, which represents the likelihood of a man-made global catastrophe, with midnight symbolising the destruction of civilisation as we know it.

In 2018, the clock was adjusted to two minutes to midnight, and in January of this year, the Bulletin of the Atomic Scientists advanced the clock forward to 100 seconds to midnight, the closest it has been to doomsday.

The Bulletin cited two simultaneous existential dangers, nuclear war and climate change, which they said was compounded by a threat multiplier, cyber-enabled information warfare, that undercuts society’s ability to respond.

“The international security situation is dire, not just because these threats exist, but because world leaders have allowed the international political infrastructure for managing them to erode,” the Bulletin said.

Former California Governor Jerry Brown, left, and former US Secretary of Defence William Perry unveil the Doomsday Clock on January 24, 2019. This year, the Doomsday Clock remains unchanged and is set at two minutes to midnight [File: Mark Wilson/Getty Images]