O N May 8, 1945, VE Day, as Europe celebrated peace at the end of six years of war, Winston Churchill was brooding on the possibility that the celebrations would soon be brutally interrupted.
For a frightening period of more than three months, neither the Allies nor the Russians knew the intentions of the other side. Churchill concluded that he must prepare for the Red Army ignoring previously-agreed frontiers and agreements in Europe.
Would Britain face waves of Communist troops who days before had been shaking the hands of their Western friends? Would Stalin stop where Hitler had or would he cross the Channel, submerging the British in a tide of blood?
Perhaps the only way of ensuring that the new order of the world was written by the West was to attack Stalin's forces before they had a chance to regroup from the chaos of their charge west.
If so, it would have to happen before the Americans withdrew the cream of their armies to concentrate on defeating Japan.
One morning in the sombre atmosphere of the Cabinet War rooms, Churchill ordered his staff to "think the unthinkable". The result was Operation Unthinkable: a putative attack on Russia by a British and American army. Churchill's War Cabinet staff officers set to work.
On May 22, only five days after the formal surrender by Admiral Doenitz of German forces, the report was handed to the Prime Minister by Lt Gen Sir Hastings Ismay, his Chief of Staff.
According to their assumptions, the Third World War could have started on July 1, 1945. The report painted a picture of the Unthinkable campaign.
"The overall or political object is to impose upon Russia the will of the United States and British Empire," the report said.
"Even though 'the will' of these two countries may be defined as no more than a square deal for Poland, that does not necessarily limit the military commitment.
"A quick success might induce the Russians to submit to our will at least for the time being; but it might not.
"That is for the Russians to decide. If they want total war, they are in a position to have it." The planners quickly dismissed the idea that the Allies should aim for a "total war", a clash with the Red Army along a front from Hamburg in the north to Trieste in the south.
The War Cabinet staff concluded that it was beyond the capabilities of the 103 divisions of Allied troops in Europe to do what Napoleon and Hitler had failed to do.
The report said the Russians would take advantage of the size of their country to defeat attempts to reduce its war-making capacity by occupation. The balance of power in Central Europe also favoured the Russians.
The report added: "To achieve the decisive defeat of Russia in a total war would require, in particular, the mobilisation of manpower to counteract their present enormous manpower resources.
"This is a very long-term project and would involve: a) the deployment in Europe of a large proportion of the vast resources of the United States. b) the re-equipment and re-organisation of German manpower and of all the Western European Allies."
If total war was unwinnable, what about a limited campaign? Churchill's team said that the best bet would be an attack by 47 British and American divisions, 14 of which would be armoured, on a two-pronged offensive, one part along the Baltic coast of Germany towards Stettin, the second further south towards Poznan, both cities being well inside Poland. It was hoped that 10 divisions of Polish troops would join the assault. They considered a possibility that most British soldiers would have found hard to swallow: the re-arming of up to 10 German divisions under a reformed German High Command. In an appendix on "German reactions to conflict between Western Allies and Russia", the planners assessed the value of having as many as 100,000 of their former enemies fighting alongside them.
"War-weariness will be the predominant feature of the attitude of the German civil population. However, ingrained fear of the Bolshevik menace and of reprisals by the Russians should make the German civil population prefer Anglo-American to Russian occupation and therefore incline it to side with the Western Allies."
There would also be vast numbers of German PoWs who would provide "a very grave source of potential disorder" if they had to be turned loose.
On the whole, the German veteran would prove to be an unreliable ally, the planners concluded, because he was consumed by an overriding "feeling that at least the war is over, although Germany has lost it".
Also to be considered were the effects on their morale of "the inevitable anomaly of changing sides . . . the known hardships of fighting on the Eastern front . . . war-weariness . . . Russian propaganda and a certain satisfaction in seeing the Allies embroiled with the Russians."
But the planners concluded that the Germans could play a useful role because of their fear of Bolshevism.
So, as infantry attacked westwards, the Royal Navy would sail along the Baltic coast, supporting the attack's left flank and harrying the Russian right almost unopposed. The RAF and the USAF would operate from bases in Denmark and northern Germany, outnumbered by the Russians, but with superior machinery. In fact, the hypothetical attack was outnumbered in every way. Opposing it were 170 Russian divisions, 30 of which were armoured. The planners also expected that the Russians would attempt sabotage in Western Europe, helped by local Communists.
Even assuming the attack broke through and the battle was won by the element of surprise and the basic superiority of the Western troops, what would happen then?
"Superior handling and air superiority might enable us to win the battle, but there is no inherent strength in our strategic position and we should, in fact, be staking everything upon the tactical outcome of one great engagement."
The Russians would retaliate and could invade Norway as far south as Trondheim, conquer Turkey and Greece in a matter of days and walk into the oilfields of "Persia" and Iraq, where 11 Red Army divisions would be opposed by three Indian brigades. The planners concluded: "If we are to embark on war with Russia, we must be prepared to be committed to a total war, which will be both long and costly.
"Our numerical inferiority on land renders it extremely doubtful whether we could achieve a limited and quick success, even if the political appreciation considered that this would suffice to gain our political object." Churchill asked Lt Gen Ismay to pass the Unthinkable report on to the Chiefs of Staff committee (COS), composed of the most senior military officers; Gen Sir Alan Brooke, Chief of the Imperial General Staff, Admiral of the Fleet Sir David Cunningham, the First Sea Lord, and Air Chief Marshal Sir Charles Portal, the Chief of the Air Staff. They replied on June 8, dismissing the report's idea that offensive action against the Russians could be taken, instead suggesting that Britain should be thinking of defence. In the month since VE Day, the Americans had begun to demobilise at a rate which alarmed the COS. It laid the bare facts before Churchill: the Russians had 264 divisions in Europe, including 36 armoured divisions, compared with 103 Allied divisions, 23 of which were armoured. America retained 64 divisions in Europe. The Soviet air force outnumbered the Allies by 11,802 in fighters and fighter-bombers, although American, British and Polish heavy bombers had a superiority of almost three to one.
The COS concluded: "It is clear from the relative strength of the respective land forces that we are not in a position to take the offensive with a view to achieving a rapid success.
"Since, however, Russian and allied land forces are in contact from the Baltic to the Mediterranean, we are bound to become involved in land operations. In support of our land forces we should have technically superior, but numerically inferior, tactical air forces.
"As regards Strategic Air Forces, our superiority in numbers and technique would be to some extent discounted by the absence of strategical targets compared to those which existed in Germany, and the necessity for using these strategic air forces to supplement our tactical air forces in support of land operations.
"Our view is, therefore, that once hostilities began, it would be beyond our power to win a quick but limited success and we should be committed to a protracted war against heavy odds.
"These odds, moreover, would become fanciful if the Americans grew weary and indifferent and began to be drawn away by the magnet of the Pacific War."
Their views clearly alerted Churchill to the fact that the best form of defence was not available to him and he should be thinking about protecting the nation from the Red Army.
He wrote to Lt Gen Ismay on June 8: "If the Americans withdraw to their zone and move the bulk of their forces back to the United States and to the Pacific, the Russians have the power to advance to the North Sea and Atlantic.
"Pray have a study made of how then we could defend our island, assuming France and the Low Countries were powerless to resist the Russian advance to the sea."
The letter concluded: "By retaining the codeword "UNTHINKABLE", the Staffs will realise this remains a precautionary study of what, I hope, is still a purely hypothetical contingency."
[In the original draft, his final words were "still a highly improbable event", but this is crossed out and the new words written in his own hand in red ink.]
On July 22, the planners produced a scheme for the defence of Britain against the Russians, setting out the disposition of forces.
It concluded Russia would not be ready to invade for several years because of Britain's supremacy at sea, but that it could mount a serious aerial bombardment by rocket and aircraft that would be far worse than the Blitz.