It is 60 years since the world entered the atomic age, with the dropping of a uranium bomb on Hiroshima. But debate still rages over whether it was justified. Nick Morrison reports.

AS he watched the flash turn from brilliant white to orange, and the fireball shoot upwards at 360ft per second before blossoming into a mushroom cloud at 30,000ft, Robert Oppenheimer recalled the words of the Hindu scripture the Bhagavad Gita: "I am become death, the destroyer of worlds."

It was just after 5.29am on July 16, 1945, below the Jemez Mountains in the New Mexico desert, and Oppenheimer, the leader of the Manhattan Project, had witnessed the dawn of the nuclear age. The Manhattan Project had been set up in 1941 to develop the United States' atomic bomb, and the device they called "The Gadget" had changed the world forever.

Three weeks later, a uranium bomb nicknamed Little Boy was dropped over Hiroshima in Japan. Tens of thousands of people died instantly, as an area half a mile in diameter was vaporised. Three days later, another atomic bomb, this one containing plutonium, was dropped over Nagasaki. Although Fat Man missed its target by more than a mile and a half, it still levelled almost half the city.

Six days after Nagasaki, Japan unconditionally surrendered, bringing an end to a conflict which cost an estimated 50 million lives worldwide.

By 1950, it is estimated that more than 300,000 people had died as a result of the bombs. Almost 200,000 more were to die from radiation illness over the following 30 years. People in Hiroshima and Nagasaki still die from radiation-linked diseases, including leukaemia and liver cancer.

As well as the human consequences, the bombs sparked an arms race which was to plunge the world into an unparalleled period of anxiety and danger. Just four years later, the Soviet Union exploded its first nuclear device, ushering in a Cold War which was to last 40 years. It was followed by Britain, France and China in developing their own nuclear arsenals. As well as the five declared nuclear states, India and Pakistan also have nuclear weapons, and Israel is generally accepted to have them too.

But the debate over the first, and so far only, hostile use of nuclear weapons is still being waged. For some, it shortened the war and avoided the loss of many more American and British servicemen. For others, it was an unnecessary atrocity inflicted on a Japan already on the verge of surrender.

The order to use the atomic bomb was taken by President Harry S Truman, after advisors told him the alternative was to lose 500,000 Americans in an invasion. But even within the highest echelons of the US government there was opposition.

Among those who believed the use of the bomb was unnecessary were Dwight D Eisenhower, Commander of Allied Forces in Europe, Truman's Chief of Staff Admiral William Leahy and General Douglas MacArthur, commander of US forces in the Pacific. According to Eisenhower: "It wasn't necessary to hit them with that awful thing."

"They took the view that it was not actually necessary to drop the bomb and that the Japanese had been trying to surrender for some time," says Kate Hudson, chairwoman of CND. She says the decision to use atomic weapons had less to do with defeating Japan than with cementing the US position in the Pacific.

"Primarily, it was for strategic, political and military reasons for the post-war period, to establish their dominance in East Asia and prevent the Soviet Union from occupying Japan, and to establish their military supremacy.

"It was not necessary as the Japanese were willing to end the war, and I believe Truman knew that. They did appear to be determined to use it, and there is a lot of evidence that it was used as a diplomatic trump card."

Japan's ultimate defeat had become increasingly obvious since the fall of the Mariana Islands in July 1944. The islands had been key to Japan's defensive perimeter; their loss meant Japan was within range of bombing runs from Pacific bases. From November 1944, Japan was hit by numerous B-29 bombing raids, destroying its industrial capacity.

A naval blockade was also preventing Japan from importing oil and other vital materials, and the surrender of Germany in May 1945 freed the Allies to concentrate their efforts in the Far East.

From June 22, the Japanese government had been trying to sue for peace, using the Soviet Union, the only member of the Allies not at war with Japan, as a mediator. But divisions over the terms of a peace treaty, and anxiety to avoid anything which appeared to be a humiliation, meant these came to naught.

But the Allies' demand for Japan's surrender, the Potsdam Proclamation of July 26, made a crucial omission, in failing to address what would happen to the Emperor when Japan surrendered. In a country where the Emperor was considered a god, the possibility that he could be stripped of his throne or even put on trial made unconditional surrender unacceptable. In the event, the Emperor was allowed to remain and Japan became a constitutional monarchy.

But Peter Zimmerman, professor of science and security at the War Studies department in King's College, London, says that the destruction of Hiroshima and Nagasaki could have saved the world from the worse fate of a nuclear war.

"The single most important thing about dropping the bombs was that we got a good idea of what a nuclear war was going to look like," he says. "Had we not used the bomb, it is impossible to believe that in the long, grim years of the Cold War, those two superpowers of the US and the Soviet Union would not have had a direct conflict. There would have been a nuclear war, and many more than two cities would have been destroyed before the escalation was brought under control."

He says that if the atomic bombs had not been dropped, the Allies would have continued with conventional bombing, killing as many people over a longer period. And there were signs that Japan was prepared to fight to the last man to avoid surrender.

Even after the second atomic bomb, it was six days before Japan surrendered. On August 9, the Soviet Union declared war on Japan and invaded Manchuria, and rumours reached the Japanese military that the next atomic bomb would be dropped on Tokyo.

It was only when Emperor Hirohito made an unprecedented intervention that the Japanese Cabinet agreed to surrender. Even then, some were dismayed, although they had no choice but to obey their emperor. War Minister Anami committed hari-kiri the day after he signed the surrender document.

But just as the debate over the only hostile use of atomic weapons continues, so the nuclear threat has remained with us.

"Even though it was apparent how horrendous it was, it didn't stop the arms race," says CND's Kate Hudson. "Even knowing how terrible they are, people still have them and still develop them. The idea of using them again is shocking."