Eighty years ago, the US detonated two nuclear weapons over the Japanese cities of Hiroshima and Nagasaki
What were the origins of the bomb?
Nuclear fission – the splitting of an atom of a heavy element, releasing vast amounts of energy – was discovered at the Kaiser Wilhelm Institute for Chemistry in Berlin in 1938. News soon spread of its vast potential. In March 1940, two refugee physicists at the University of Birmingham, Rudolf Peierls and Otto Frisch, issued a memorandum to the UK government, predicting that an airborne atomic "superbomb" capable of destroying "the centre of a big city" could be created. It was vital, they warned, to beat the Nazis to its development. The British nuclear programme – codenamed Tube Alloys – started in 1941, but the UK lacked the necessary resources to build a bomb; British expertise was subsumed into the US nuclear programme.
What resources did the US have?
The Manhattan Project, as it was later codenamed, began modestly in 1939, but from 1942 it grew to employ around 130,000 scientists, engineers and support staff, at a cost of over $2 billion (or about $39 billion in today's money). Fewer than a dozen knew they were working on an atomic bomb specifically. Most of the money was spent producing fissile materials – enriched uranium and plutonium; less than 10% was spent developing the weapons at the Los Alamos facility in New Mexico. The Trinity Test, the first-ever detonation of an atomic bomb, took place on 16 July 1945.
What triggered the bomb's use?
This is a controversial question, but the short answer is: the need to end the Second World War. By July 1945, the bomb's first intended target, Germany, had surrendered, but Japan was fighting on, and while US victory seemed inevitable, the Pacific war had taken the lives of more than 115,000 US soldiers. During the battle for the outlying Japanese island of Okinawa, launched in April 1945, almost the entire Japanese defending force of more than 90,000 perished, along with as many as 100,000 civilians. More than 12,000 US servicemen were killed in action; kamikaze pilots sank some 26 ships. It convinced the US leadership that an invasion of Japan's home islands would be immensely costly.
Why didn't Japan surrender?
It was Allied policy to demand unconditional surrender. This was reiterated by the Potsdam Declaration of 26 July 1945, in which the US, Great Britain and China stated that if Japan did not accept these terms, it faced "prompt and utter destruction". Japan's military leaders felt this was too "dishonourable", and were concerned about the future status of Emperor Hirohito, who was regarded as an incarnate god, and whether he would remain in power. PM Kantaro Suzuki rejected the declaration – somewhat ambiguously – and President Truman authorised the use of the atomic bomb at any time after 3 August; the US had enough fuel for three bombs.
How was the first bomb dropped?
At 2.45am on 6 August, a B-29 bomber – named Enola Gay after the mother of the pilot, Colonel Paul Tibbets – left the Mariana Islands. The weather was clear, and en route it was confirmed that the city of Hiroshima, home to 350,000 and a large army base, would be the target for its single bomb – codename Little Boy – which contained 64kg of uranium, with the power of some 15,000 tons of TNT. At 8.15am it was dropped from 31,600 feet over the city; 44 seconds later, as Enola Gay retreated at full speed, it detonated at 1,968 feet above ground level. A blinding flash lit up Hiroshima, as the bomb's "burst point" reached more than one million degrees celsius, and sent a mushroom cloud billowing some 50,000 feet into the air.
What was its impact at ground level?
An atomic shockwave tore through the city, incinerating everything within a 1.5-mile radius; the ensuing firestorm destroyed virtually everything within a 4.5-mile radius. "It was like no ordinary fire," a crew member later recalled. "It contained a dozen colours, all of them blindingly bright." Some 60,000 buildings were instantly destroyed. The death tolls are contested, but between 90,000 and 160,000 people died in Hiroshima – about half on the first day, the rest from radiation sickness and burns within a year. It was, said one survivor, Sunao Tsuboi, "a living hell on Earth. I saw a schoolgirl with her eye hanging out of its socket. People looked like ghosts, bleeding and trying to walk, before collapsing. Some had lost limbs. There were charred bodies everywhere… The smell of burning flesh was overpowering."
Did it force Japan's surrender?
Not immediately. President Truman confirmed that the bomb had been dropped, and warned: "If they 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." On 9 August, another B-29 – Bockscar – was loaded with an atomic bomb, this time made with plutonium: it was codenamed Fat Man. Its initial target was the city of Kokura, but its view from the air was obscured by smoke, so nearby Nagasaki was chosen instead. The second – and final – atomic bomb ever to be dropped on humans exploded at 11.02am on 9 August 1945. Between 40,000 and 70,000 people were killed in the immediate aftermath; the city was somewhat protected by its position in a valley. Six days after Nagasaki, Hirohito made his first-ever broadcast to the Japanese people, to announce the country's surrender. (Even then, the Supreme War Council had been split 3 to 3; Hirohito had the casting vote.) It was signed aboard the USS Missouri on 2 September, bringing the Second World War to a close.
A necessary evil or "atomic blackmail"?
"My God, what have we done?" wrote Enola Gay's co-pilot Robert Lewis in a log composed while flying back from Hiroshima. His sense of a foreboding at the scale of the destruction was widely shared. However, for many years, in the US and Britain, Truman's justification for dropping the bombs – "to end the war quickly and save countless lives" – was accepted almost without question. Most of America's political and military leadership shared his view. In May 1945, the Interim Committee, which guided nuclear policy, had concluded by consensus (though one member later dissented) "that the bomb should be used against Japan as soon as possible; that it be used on a war plant surrounded by workers' homes; and that it be used without prior warning". The US was, after all, fighting a fanatical and brutal enemy. And the bombing of enemy cities had been policy for years: some 100,000 people were killed in the firebombing of Tokyo on 9-10 March 1945.
In the 1960s, revisionist historians began to question every aspect of Truman's account. Had the bombs ended the war? Or had the Soviet Union's declaration of war against Japan on 8 August – and assurances given that Hirohito would not be punished – played a bigger role? Stalin had called the decision "atomic blackmail": a warning to the Soviet Union. It was argued that Truman had more humane alternatives: blockading Japan, using the bomb on an exclusively military target, or negotiating more acceptable surrender terms. Today, historical opinion is still sharply divided.
