Researching my book, Operation Big – The Race to Stop Hitler’s A-Bomb, forced me to revise my view of the biggest event of the 20th Century – the dropping of the nuclear bomb of Hiroshima on 6 August 1945.

I had been brought up to believe that the atomic bomb was dropped on Japan to force Emperor Hirohito into surrender and end the Second World War. I began to revise my views when I read R V Jones’s highly-readable memoir, Most Secret War, and this line used on the back cover of Operation Big: ‘We ourselves were almost awestruck, not so much at the power of the Bomb, for this we had expected, but because the Americans had used it with so little notice.’

R V Jones said that British intelligence knew the Japanese were putting out feelers for surrender when the Americans dropped the first of two nuclear bombs on them. I dug deeper into the American archives – many can be accessed online - and found a more disturbing story at the core of Operation Big and the Alsos Mission led by Colonel Boris T Pash to capture the leading nuclear scientists in the Third Reich in the dying days of the war.

That is why I called the last chapter ‘The Dirty Secret’. It became glaringly obvious as I delved into the archives – the Pash papers at the Hoover Institute, Stanford University in California with the help of my researcher Dr Camilla Lindan, R V Jones’s papers held at the Churchill Archives at Churchill College Cambridge and the Cabinet papers of Sir Winston Churchill in the National Archives in Kew - that there was more to the Alsos Mission than the capture of Hitler’s nuclear scientists.

Farm Hall as drawn by Erich Bagge while he was a ‘guest’ at Farm Hall.

The fact that they were airlifted by MI6 to Farm Hall in Godmanchester, a beautiful bucolic slice of England by the water meadows of Cambridgeshire – Rupert Brooke wrote his elegiac poem The Old Vicarage about Granchester Meadows a few miles away – was always going to make the headlines. But the underlying story was more sinister. Facts kept nagging away: I discovered Sam Goudsmit, the scientific head of the Alsos Mission reported back to Washington as early as November 1944 that Hitler’s physicists had not built an atomic bomb.

In Pash’s memoir, The Alsos Mission, backed by his archives at the Hoover Institute, Pash recalled the breakthrough came when they seized documents in Strasbourg and Goudsmit shouted: ‘We’ve got it!’

‘I know we have it,’ said Pash. ‘But do they?’

Goudsmit’s eyes were wide with excitement. ‘No, no!’ he said. ‘That’s it. They don’t.’

Pash recorded: ‘It was our Strasbourg operation which disclosed that it was unlikely that the Nazis could unleash an atom bomb in the near future. Thus Alsos exploded the Nazi super-weapon myth that had so alarmed Allied leaders. The fact that a German atom bomb was not an immediate threat was probably the most significant single piece of military intelligence developed throughout the war.’

Pash claimed Alsos had “exploded the biggest intelligence bombshell of the war” in November 1944 – a full seven months before the German scientists arrived in Godmanchester. But if so, why I wondered did Pash and his team of US intelligence officers and soldiers in Jeeps – they were accused of operating as if they were in the “Wild West” - continue the hunt for the ten German scientists across the Rhine, into Germany and all the way to Heisenberg’s hideaway in the Bavarian Alps?

Colonel Boris T. Pash (right) on Operation Big in Hechingen with Sergeant Holt (middle) and Corporal Brown (left).

It is true Pash and Goudsmit had to be certain that they were right, that there was no Nazi A-bomb, but there was a bigger picture emerging that was exercising their chiefs back in Washington, led by the uncompromising General Leslie Groves, the Army engineer who headed the construction of Manhattan Project, the massive industrial effort to build the world’s first nuclear bombs.

Groves operated on the principle that if the US could do it, so could the Germans. But he was also determined to stop the German physicists falling into Soviet hands. His biggest fear – now that the threat of a Nazi bomb could be discounted – was that the Soviets would gain the know-how from the Germans that had been achieved by the Americans over the past three years of hard work in the laboratories of the Manhattan Project.

Groves in his own memoir, Now It Can be Told, makes clear he ordered the bombing of Auergesellschaft Works in Oranienburg 15 miles north of Berlin on 15 March 1945 to stop uranium ore being seized by the Russians because it was in the sector allocated to the Soviet Union at the Yalta conference of the Big Three. And it was not just the Russians Groves opposed. Groves did not trust the British, and particularly distrusted the French because their lead physicist in Paris, Joliot-Curie was a Communist. ‘Joliot convinced me that nothing that might be of interest to the Russians should ever be allowed to fall into French hands.’

The Alsos team dismantling the German atomic pile at Haigerloch – portly Lieutenant Commander Eric Welsh stands on the rim handing out graphite blocks. Wing Commander Rupert Cecil is in the foreground.

Against that background, the focus of the Alsos Mission and its conclusion – Operation Big – switched from Hitler and the Nazi threat to combatting the Russian threat. By the time Hitler’s Uranverein (Uranium Club) arrived at their five-star country house hotel in Godmanchester, Groves and the chiefs in Washington were preparing for the Cold War, and what they could do to regain some of the influence they had surrendered to the Soviet advance across Europe as Josef Stalin’s Red Army swept into Germany from the East, making huge territorial gains which would be described by Churchill as the “Iron Curtain”.

In the Truman administration at the White House, the bomb was seen as the answer. Truman was completely unapologetic about his decision taken after the Potsdam conference with Churchill and Stalin in 1945 where he had been informed that the “Trinity” test of the new weapon had been a success. Truman convened a secret meeting of his top advisers – Byrnes, Secretary of State, Stimson, Secretary of War, Eisenhower and Marshall. ‘I asked General Marshall what it would cost in lives to land on the Tokyo plain and other places in Japan. It was his opinion that such an invasion would cost at a minimum one quarter of a million casualties, and might cost as much as a million, on the American side alone, with an equal number of the enemy…Dropping the bombs ended the war, saved lives and gave free nations a chance to face the facts.’ (Letter 12 January 1953 Truman to Professor James L Cate).

The drawing room at Farm Hall where the scientists heard the news about the detonation at Hiroshima.

But Truman was being “economical with the actualite” as the late Tory defence minister Alan Clark said in a different context. The truth is Truman, in addition to ending the war against Japan, also wanted to demonstrate to the Russians that America had become the first truly great super power because it possessed a bomb capable of destruction on a hitherto unimaginable scale. He did not know that thanks to Klaus Fuchs, the Soviet agent embedded in the Manhatten Project, Josef Stalin knew more than he did about the nuclear bomb.

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Colin Brown's new paperback version of his book Operation Big: The Race to Stop Hitler's A-Bomb is available now.