On 6 August 1945 – 67 years ago today – a control operator at the Japanese Broadcasting Corporation noticed that there was no signal from Radio Hiroshima. It had, seemingly, gone off air. Telephone calls couldn't reach the city centre either. There was a simple reason for this – the city centre wasn't there any more.

At 8.15am an American B-29 bomber had dropped an atom bomb on Hiroshima. People were literally vaporised by a light '"brighter than a thousand suns". A firestorm and 600mph winds sucked the remaining air out of the downtown district. Soon a mushroom cloud spiralled into the stratosphere, and under it 140,000 civilians lay dead.

As Japan absorbed what had happened and its rulers prepared a formal protest at the new weapon, a second strike was prepared. The target was Kokura but on 9 August, it was obscured by fog so Nagasaki was hit. The death toll was 70,000. On 12 August Japan's Emperor, Hirohito, said surrender was inevitable. The war was over, but the bomb debate was just beginning.

Apologists for these events have used two arguments. These attacks were necessary because Japan wouldn't surrender without them, and because a land invasion against Japan's disciplined troops would have caused 300,000 US casualties or more. The bombing also kept the Soviets out of Japan and helped speed the end of the war. This thought now dominates – anyone disagreeing is "a soft peacenik". No one objected to the A-Bomb's use in 1945, we are told. No one who knew the score amongst the military high-ups. There was no alternative.

But the argument that no one in the know objected is a fallacy. General Eisenhower opposed it, "Japan was already defeated… dropping the bomb was completely unnecessary." The Pacific Fleet commander Admiral Nimitz agreed: "The Japanese had, in fact, already sued for peace. The atomic bomb played no decisive part, from a purely military point of view, in their defeat." Admiral Leahy, President Truman's Chief of Staff, concurred: the atomic attacks were "of no material assistance in our war against Japan. The Japanese were already ready to surrender…"

By the spring of 1945 Japan was faltering. Germany surrendered in May and since April US aircraft had roamed almost at will over Japan. Heavy bombing raids using dozens of B-29s were met with token resistance, and the firebombing of Tokyo had not been seriously opposed. A sea blockade had decimated imports.

During this time Japan put out peace feelers: on 25 July Japan tried to get envoys to Russia, carrying Imperial letters which read, in part: "His Majesty… mindful of the fact that the present war daily brings greater evil and sacrifice of the peoples… desires from his heart that it may be quickly terminated. But as long as England and the US insist upon unconditional surrender the Japanese Empire has no alternative but to fight on… for the honour and existence of the Motherland …"

These feelers were rebuffed by the US demand for unconditional surrender. But this was unacceptable to Japan, for it could mean that Hirohito –seen as semi-divine – could be put on trial. In mid-1945 The Washington Post kept asking why Truman was demanding unconditional surrender while granting that a condition could swiftly end hostilities. In July, Time wondered whether the answer was some "deep secret" while the United States News confirmed, days after Hiroshima, that "competent testimony exists to prove that Japan was seeking to surrender many weeks before the atomic bomb…"

And, of course, post-Nagasaki, the US did grant the condition that the Emperor be left alone. So if America could agree to this in August, why not in July or even June? Why not end the war earlier? US stubbornness only makes sense if it's seen for what it really is: an excuse to delay peace long enough to test the bomb on real cities. Which is why previous heavy bombing raids had always spared the first atomic targets: Hiroshima, Kokura, Niigata and Kyoto.

These centres had been left virtually free of heavy bombing for just this purpose, so the effect of atomic destruction could be seen on "virgin targets". There is no other reason why military-industrial cities like Kokura, Hiroshima and Niigata were ignored. Nagasaki was not on the "death list" – it was hit by a small raid on 1 August – but it replaced Kyoto when it was decided that the latter should be spared. In 1958 the rightist National Review admitted that "the main purpose of using the atomic bombs on Japan was not military, but diplomatic, and that the real target was not Japan but Russia…"

So where does this leave us? US demands for Japanese unconditional surrender were always unrealistic – and deliberately so. This intentionally prolonged the war for the sole purpose of testing the atomic bomb on real cities. These attacks killed thousands, as did delaying the peace. This also allowed Stalin to take Manchuria, and Soviet triumph there inadvertently helped Chairman Mao to seize China, a move that later killed millions. This inevitably led to the Korean War. It is difficult to think of a less desirable set of consequences.