The world changed forever when a US bomber dropped the first atomic bomb on the Japanese city of Hiroshima 70 years ago.

The Americans said they took the drastic step to put an early end to World War II and save the lives of hundreds of thousands of US soldiers, but this official narrative is now being overturned.

On August 6, 1945 the world's first atomic bomb exploded over Hiroshima, wiping out the city centre and killing about 140,000 people by the years' end.

Keiko Ogura was eight-years-old at the time and only 2.4 kilometres from the hypocentre.

She remembers being engulfed in flames.

"A flash of light and the blast slammed me to the ground and I lost consciousness," she said.

"I woke up, it was dark and everyone was crying."

Keiko said the atomic bombing of Hiroshima and another at Nagasaki three days later, which killed 70,000 more, were war crimes.

Many historians say the bombings did not lead to the Japanese surrender, and the Soviet declaration of war on Japan two days later was a bigger shock.

It put an end to any hope the Soviets would negotiate a favourable surrender for Japan.

The severely-weakened Japanese Imperial army had no capacity to fight the Soviets on a second front in China and Northern Japan.

Japanese historian Yuki Tanaka said the country had no choice because the Soviets would have killed Emperor Hirohito, seen as the heart and soul of imperial Japan.

"The Soviet Union would demolish the emperor system and they would execute the emperor as well as all members of the royal family," he said.

New bomb not responsible for Japan's surrender

America believed the shock and awe of the devastating power of the new bombs would force Japan into surrender, but experts say inside Japan it was viewed differently.

The Americans had already destroyed 66 Japanese cities with a massive fire bombing campaign.

In just one night, 100,000 civilians were killed in Tokyo.

Tokyo's Temple University director of Asian Studies Jeffery Kingston said the new bombs would not have had the impact the Americans would have hoped.

"If you look at it from the perspective of the Japanese military, it doesn't really make a big difference whether people are dying from fire bombing or atomic bombs ... it is [just] two additional city centres that are destroyed," he said.

The atomic bombings probably did play a part in averting a bloody ground invasion and saving thousands of US lives, but historians like Dr Kinston said the bombs were also about sending a message to the Soviets.

"We have this incredible new weapon, we have a monopoly on it and we are going to emerge as the strongest superpower. In a sense, this was the opening salvo of the Cold War," he said.

The crew of the B-29 bomber Enola Gay navigator Major Theodore Van Kirk (L), pilot Colonel Paul Tibbets and bombardier Major Thomas Ferebee after dropping the first atomic bomb on Hiroshima on August 6, 1945. ( AFP: US Air Force )

Survivor's call for future free of nuclear weapons

On the eve of 70th anniversary, the children of Hiroshima sing for a future free of nuclear weapons, but today more countries than ever have the bomb.

America's atomic attacks on Japan started a nuclear arms race which bought the world to the brink of destruction.

Hiroshima survivor Keiko Ogura wants people to come and see for themselves.

"Some people in the world still do not understand the cruelty of nuclear weapons, and that they are absolute evil. This surprises me. I want them to come to Hiroshima and Nagasaki," she said.

Most historians agree the official version from the US government — that the bombs were dropped to force an early surrender and saved up to a million American lives from a bloody invasion — is far too simplistic.

Mr Tanaka said it was a criminal act under international law.

"Also, morally it was wrong. So they have to find some non-legal arguments to self-justify the conduct, that they annihilated 210,000 civilians," he said.

The debate in academic circles now is how much other factors came into play in America becoming the first and still the only nation to drop the atomic bomb in warfare.

The issue will be debated on Lateline at 10.45pm